manuals: document "mime-xdg" class and MIME_XDG_PACKAGES

(From yocto-docs rev: 4415d95358497b23f0a7b10f9ee31203ccc01eff)

Signed-off-by: Michael Opdenacker <michael.opdenacker@bootlin.com>
Signed-off-by: Steve Sakoman <steve@sakoman.com>

documentation/migration-guides/migration-3.1.rst

@@ -238,7 +238,7 @@ Warnings will now be shown at ``do_package_qa`` time in the following
 circumstances:
 
 -  A recipe installs ``.desktop`` files containing ``MimeType`` keys but
-   does not inherit the new ``mime-xdg`` class
+   does not inherit the new :ref:`mime-xdg <ref-classes-mime-xdg>` class
 
 -  A recipe installs ``.xml`` files into ``${datadir}/mime/packages``
    but does not inherit the :ref:`mime <ref-classes-mime>` class

documentation/ref-manual/classes.rst

@@ -1577,6 +1577,27 @@ The ``mime`` class generates the proper post-install and post-remove
 These scriptlets call ``update-mime-database`` to add the MIME types to
 the shared database.
 
+.. _ref-classes-mime-xdg:
+
+``mime-xdg.bbclass``
+====================
+
+The :ref:`mime-xdg <ref-classes-mime-xdg>` class generates the proper
+post-install and post-remove (postinst/postrm) scriptlets for packages
+that install ``.desktop`` files containing ``MimeType`` entries.
+These scriptlets call ``update-desktop-database`` to add the MIME types
+to the database of MIME types handled by desktop files.
+
+Thanks to this class, when users open a file through a file browser
+on recently created images, they don't have to choose the application
+to open the file from the pool of all known applications, even the ones
+that cannot open the selected file.
+
+If you have recipes installing their ``.desktop`` files as absolute
+symbolic links, the detection of such files cannot be done by the current
+implementation of this class. In this case, you have to add the corresponding
+package names to the :term:`MIME_XDG_PACKAGES` variable.
+
 .. _ref-classes-mirrors:
 
 ``mirrors.bbclass``

documentation/ref-manual/qa-checks.rst

@@ -590,7 +590,7 @@ Errors and Warnings
 - ``package contains desktop file with key 'MimeType' but does not inhert mime-xdg: <packagename> path '<file>' [mime-xdg]``
 
     The specified package contains a .desktop file with a 'MimeType' key
-    present, but does not inherit the :ref:`ref-classes-mime-xdg`
+    present, but does not inherit the :ref:`mime-xdg <ref-classes-mime-xdg>`
     class that is required in order for that to be activated. Either add
     ``inherit mime`` to the recipe or remove the files at the
     :ref:`ref-tasks-install` step if they are not needed.

documentation/ref-manual/variables.rst

@@ -4852,6 +4852,13 @@ system and gives an overview of their function and contents.
       The revision currently checked out for the OpenEmbedded-Core layer (path
       determined by :term:`COREBASE`).
 
+   :term:`MIME_XDG_PACKAGES`
+      The current implementation of the :ref:`mime-xdg <ref-classes-mime-xdg>`
+      class cannot detect ``.desktop`` files installed through absolute
+      symbolic links. Use this setting to make the class create post-install
+      and post-remove scripts for these packages anyway, to invoke the
+      ``update-destop-database`` command.
+
    :term:`MIRRORS`
       Specifies additional paths from which the OpenEmbedded build system
       gets source code. When the build system searches for source code, it