Document the magic gettext macros in gi18n.h

These symbols are barely introspectable, which means they have to be
collected into the existing i18n content document.

Fixes: #3361

docs/reference/glib/i18n.md

@@ -47,3 +47,103 @@ The [gettext
 manual](http://www.gnu.org/software/gettext/manual/gettext.html#Maintainers)
 covers details of how to integrate gettext into a project’s build system and
 workflow.
+
+## Macros
+
+GLib provides various convenience C pre-processor macros that make it easy
+for tools like [`xgettext`](https://www.gnu.org/software/gettext/manual/html_node/xgettext-Invocation.html)
+to extract translatable strings from the source of a library or an
+application. These macros are defined when including `<glib/gi18n.h>` or
+`<glib/gi18n-lib.h>`.
+
+`_(String)`
+:   Marks a string for translation. The string will be replaced by its
+    translation at run time, if any exists; otherwise, it will be passed
+    as is.
+
+
+`N_(String)`
+:   Marks a string for translation. Unlike `_()`, this macro will not
+    replace the string with its translation; this is useful when the
+    translatable string is inside a `struct` or array declaration, for
+    instance:
+
+        static const char *messages[] = {
+          N_("some very meaningful message"),
+          N_("and another one"),
+        };
+
+    It is the responsibility of the developer to call `gettext()` when
+    using the string, e.g.:
+
+        g_print ("%s", idx >= G_N_ELEMENTS (messages)
+                 ? _("default message")
+                 : gettext (messages[idx]));
+
+
+`C_(Context, String)`
+:   Uses `gettext` to get the translation for `String`. `Context` is
+    used as a context. This is mainly useful for short strings which
+    may need different translations, depending on the context in which
+    they are used. For instance:
+
+        label1 = C_("Navigation", "Back");
+        label2 = C_("Body part", "Back");
+
+    If you are using the `C_()` macro, you need to make sure that you
+    pass `--keyword=C_:1c,2` to `xgettext` when extracting messages.
+    This only works with a version of GNU gettext newer than 0.15.
+
+    This macro is available since GLib 2.16
+
+
+`NC_(Context, String)`
+:   Only marks a string for translation, with context. Similar to `N_()`,
+    but allows you to add a context to the translatable string, for
+    instance:
+
+        static const char *messages[] = {
+          NC_("some context", "some very meaningful message"),
+          NC_("some context", "and another one")
+        };
+
+    It is the responsibility of the developer to call [func@GLib.dpgettext2]
+    when using the string, e.g.:
+
+        g_print ("%s", idx >= G_N_ELEMENTS (messages)
+                 ? g_dpgettext2 (NULL, "some context", "a default message")
+                 : g_dpgettext2 (NULL, "some context", messages[idx]);
+
+    If you are using the `NC_()` macro, you need to make sure that you pass
+    `--keyword=NC_:1c,2` to `xgettext` when extracting messages. This only
+    works with a version of GNU gettext newer than 0.15. Intltool has support
+    for the `NC_()` macro since version 0.40.1.
+
+    This macro is available since GLib 2.18
+
+`Q_(String)`
+:   `Like _()`, but handles context inside translatable string. This has
+    the advantage that the string can be adorned with a prefix to guarantee
+    uniqueness and provide context to the translator.
+
+    The `String` is made of two parts, separated by `|` character. The
+    leading part is the prefix, which must not be translated; the trailing
+    part is the translatable message.
+
+    One use case given in the gettext manual is GUI translation, where one
+    could e.g. disambiguate two "Open" menu entries as "File|Open" and
+    "Printer|Open". Another use case is the string "Russian" which may
+    have to be translated differently depending on whether it's the name
+    of a character set or a language. This could be solved by using
+    "charset|Russian" and "language|Russian".
+
+    See also the `C_()` macro for a different way to mark up translatable
+    strings with context.
+
+    If you are using the `Q_()` macro, you need to make sure that you pass
+    `--keyword=Q_` to `xgettext` when extracting messages. If you are using
+    a version of GNU gettext newer than 0.15, you can also use `--keyword=Q_:1g`
+    to let xgettext split the context string off into a `msgctxt` line in
+    the po file.
+
+    This macro is available since GLib 2.4