docs: Move the GApplicationCommandline SECTION

Move the contents to the struct docs.

Helps: #3037

gio/gapplicationcommandline.c

@@ -40,15 +40,13 @@
 #endif
 
 /**
- * SECTION:gapplicationcommandline
- * @title: GApplicationCommandLine
- * @short_description: A command-line invocation of an application
- * @include: gio/gio.h
- * @see_also: #GApplication
+ * GApplicationCommandLine:
  *
- * #GApplicationCommandLine represents a command-line invocation of
- * an application.  It is created by #GApplication and emitted
- * in the #GApplication::command-line signal and virtual function.
+ * `GApplicationCommandLine` represents a command-line invocation of
+ * an application.
+ *
+ * It is created by [struct@Gio.Application] and emitted
+ * in the [signal@Gio.Application.command-line] signal and virtual function.
  *
  * The class contains the list of arguments that the program was invoked
  * with. It is also possible to query if the commandline invocation was
@@ -56,20 +54,20 @@
  * invocation) or remote (ie: some other process forwarded the
  * commandline to this process).
  *
- * The GApplicationCommandLine object can provide the @argc and @argv
- * parameters for use with the #GOptionContext command-line parsing API,
- * with the g_application_command_line_get_arguments() function. See
+ * The `GApplicationCommandLine` object can provide the @argc and @argv
+ * parameters for use with the [struct@Glib.OptionContext] command-line parsing API,
+ * with the [method@Gio.ApplicationCommandLine.get_arguments] function. See
  * [gapplication-example-cmdline3.c][gapplication-example-cmdline3]
  * for an example.
  *
  * The exit status of the originally-invoked process may be set and
  * messages can be printed to stdout or stderr of that process. The
- * lifecycle of the originally-invoked process is tied to the lifecycle
+ * life-cycle of the originally-invoked process is tied to the lifecycle
  * of this object (ie: the process exits when the last reference is
  * dropped).
  *
- * The main use for #GApplicationCommandLine (and the
- * #GApplication::command-line signal) is 'Emacs server' like use cases:
+ * The main use for `GApplicationCommandLine` (and the
+ * [signal@Gio.Application.command-line] signal) is 'Emacs server' like use cases:
  * You can set the `EDITOR` environment variable to have e.g. git use
  * your favourite editor to edit commit messages, and if you already
  * have an instance of the editor running, the editing will happen
@@ -78,11 +76,12 @@
  * does not return until the editing is done.
  *
  * Normally, the commandline is completely handled in the
- * #GApplication::command-line handler. The launching instance exits
+ * [signal@Gio.Application.command-line handler]. The launching instance exits
  * once the signal handler in the primary instance has returned, and
  * the return value of the signal handler becomes the exit status
  * of the launching instance.
- * |[<!-- language="C" -->
+ *
+ * ```c
  * static int
  * command_line (GApplication            *application,
  *               GApplicationCommandLine *cmdline)
@@ -104,13 +103,15 @@
  *
  *   return 0;
  * }
- * ]|
+ * ```
+ *
  * The complete example can be found here:
  * [gapplication-example-cmdline.c](https://gitlab.gnome.org/GNOME/glib/-/blob/HEAD/gio/tests/gapplication-example-cmdline.c)
  *
  * In more complicated cases, the handling of the commandline can be
  * split between the launcher and the primary instance.
- * |[<!-- language="C" -->
+ *
+ * ```c
  * static gboolean
  *  test_local_cmdline (GApplication   *application,
  *                      gchar        ***arguments,
@@ -156,18 +157,19 @@
  *
  *   ...
  * }
- * ]|
+ * ```
+ *
  * In this example of split commandline handling, options that start
  * with `--local-` are handled locally, all other options are passed
- * to the #GApplication::command-line handler which runs in the primary
+ * to the [signal@Gio.Application.command-line] handler which runs in the primary
  * instance.
  *
  * The complete example can be found here:
  * [gapplication-example-cmdline2.c](https://gitlab.gnome.org/GNOME/glib/-/blob/HEAD/gio/tests/gapplication-example-cmdline2.c)
  *
- * If handling the commandline requires a lot of work, it may
- * be better to defer it.
- * |[<!-- language="C" -->
+ * If handling the commandline requires a lot of work, it may be better to defer it.
+ *
+ * ```c
  * static gboolean
  * my_cmdline_handler (gpointer data)
  * {
@@ -197,10 +199,11 @@
  *
  *   return 0;
  * }
- * ]|
+ * ```
+ *
  * In this example the commandline is not completely handled before
- * the #GApplication::command-line handler returns. Instead, we keep
- * a reference to the #GApplicationCommandLine object and handle it
+ * the [signal@Gio.Application.command-line] handler returns. Instead, we keep
+ * a reference to the `GApplicationCommandLine` object and handle it
  * later (in this example, in an idle). Note that it is necessary to
  * hold the application until you are done with the commandline.
  *
@@ -208,13 +211,6 @@
  * [gapplication-example-cmdline3.c](https://gitlab.gnome.org/GNOME/glib/-/blob/HEAD/gio/tests/gapplication-example-cmdline3.c)
  */
 
-/**
- * GApplicationCommandLine:
- *
- * #GApplicationCommandLine is an opaque data structure and can only be accessed
- * using the following functions.
- */
-
 /**
  * GApplicationCommandLineClass:
  *