docs: Fill in gshell long description

glib/gshell.c

@@ -37,7 +37,15 @@
  * SECTION:shell
  * @title: Shell-related Utilities
  * @short_description: shell-like commandline handling
- **/
+ *
+ * GLib provides the functions g_shell_quote() and g_shell_unquote()
+ * to handle shell-like quoting in strings. The function g_shell_parse_argv()
+ * parses a string similar to the way a POSIX shell (/bin/sh) would.
+ *
+ * Note that string handling in shells has many obscure and historical
+ * corner-cases which these functions do not necessarily reproduce. They
+ * are good enough in practice, though.
+ */
 
 /**
  * G_SHELL_ERROR: