its: Improve comments in *.its files.

* gettext-tools/its/glade2.its: Improve comments.
* gettext-tools/its/gtkbuilder.its: Likewise.
* gettext-tools/its/gsettings.its: Likewise.
* gettext-tools/its/metainfo.its: Likewise.

gettext-tools/its/glade2.its

@@ -19,6 +19,12 @@
 <its:rules xmlns:its="http://www.w3.org/2005/11/its"
            xmlns:gt="https://www.gnu.org/s/gettext/ns/its/extensions/1.0"
            version="2.0">
+
+  <!-- Documentation of the Glade 2 XML file format:
+       https://download.gnome.org/sources/libglade/2.6/libglade-2.6.4.tar.gz
+       file libglade-2.6.4/doc/html/libglade-dtd.html
+  -->
+
   <its:translateRule selector="/glade-interface" translate="no"/>
   <its:translateRule
       selector="/glade-interface//property[@translatable = 'yes'] |

gettext-tools/its/gsettings.its

@@ -19,6 +19,11 @@
 <its:rules xmlns:its="http://www.w3.org/2005/11/its"
            xmlns:gt="https://www.gnu.org/s/gettext/ns/its/extensions/1.0"
            version="2.0">
+
+  <!-- Documentation of the GSettings XML file format:
+       https://docs.gtk.org/gio/class.Settings.html
+  -->
+
   <its:translateRule selector="/schemalist" translate="no"/>
 
   <its:translateRule selector="//summary | //description" translate="yes"/>

gettext-tools/its/gtkbuilder.its

@@ -19,8 +19,16 @@
 <its:rules xmlns:its="http://www.w3.org/2005/11/its"
            xmlns:gt="https://www.gnu.org/s/gettext/ns/its/extensions/1.0"
            version="2.0">
+
+  <!-- Documentation of the GtkBuilder XML file format:
+       https://docs.gtk.org/gtk3/class.Builder.html
+       https://docs.gtk.org/gtk4/class.Builder.html
+       A better documentation does not exist; see
+       https://gitlab.gnome.org/GNOME/gtk/-/issues/4039 .
+  -->
+
   <its:translateRule selector="/interface" translate="no"/>
-  <!-- https://developer-old.gnome.org/gtk4/stable/GtkBuilder.html says about
+  <!-- https://docs.gtk.org/gtk4/class.Builder.html says about
        boolean attribute values:
        "strings like “TRUE”, “t”, “yes”, “y”, “1” are interpreted as TRUE,
         strings like “FALSE”, “f”, “no”, “n”, “0” are interpreted as FALSE"

gettext-tools/its/metainfo.its

@@ -19,6 +19,11 @@
 <its:rules xmlns:its="http://www.w3.org/2005/11/its"
            xmlns:gt="https://www.gnu.org/s/gettext/ns/its/extensions/1.0"
            version="2.0">
+
+  <!-- Documentation of the *.metainfo.xml file format:
+       https://www.freedesktop.org/software/appstream/docs/chap-Quickstart.html
+  -->
+
   <its:translateRule selector="/component" translate="no"/>
   <its:translateRule selector="/component/name |
                                /component/summary |