ref-manual: add meson class and variables

Backported from the master branch

(From yocto-docs rev: 266540ffdf84df14ebde374927e6e8ddd8ee688e)

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

documentation/ref-manual/variables.rst

@@ -2345,6 +2345,20 @@ system and gives an overview of their function and contents.
       variable specifies additional configuration options you want to pass
       to the ``scons`` command line.
 
+   :term:`EXTRA_OEMESON`
+      Additional `Meson <https://mesonbuild.com/>`__ options. See the
+      :ref:`ref-classes-meson` class for additional information.
+
+      In addition to standard Meson options, such options correspond to
+      `Meson build options <https://mesonbuild.com/Build-options.html>`__
+      defined in the ``meson_options.txt`` file in the sources to build.
+      Here is an example::
+
+         EXTRA_OEMESON = "-Dpython=disabled -Dvalgrind=disabled"
+
+      Note that any custom value for the Meson ``--buildtype`` option
+      should be set through the :term:`MESON_BUILDTYPE` variable.
+
    :term:`EXTRA_USERS_PARAMS`
       When inheriting the :ref:`extrausers <ref-classes-extrausers>`
       class, this variable provides image level user and group operations.
@@ -4794,6 +4808,17 @@ system and gives an overview of their function and contents.
    :term:`MAINTAINER`
       The email address of the distribution maintainer.
 
+   :term:`MESON_BUILDTYPE`
+      Value of the Meson ``--buildtype`` argument used by the
+      :ref:`ref-classes-meson` class. It defaults to ``debug`` if
+      :term:`DEBUG_BUILD` is set to "1", and ``plain`` otherwise.
+
+      See `Meson build options <https://mesonbuild.com/Builtin-options.html>`__
+      for the values you could set in a recipe. Values such as ``plain``,
+      ``debug``, ``debugoptimized``, ``release`` and ``minsize`` allow
+      you to specify the inclusion of debugging symbols and the compiler
+      optimizations (none, performance or size).
+
    :term:`METADATA_BRANCH`
       The branch currently checked out for the OpenEmbedded-Core layer (path
       determined by :term:`COREBASE`).