DOC: add documentation for the custom meson config setting and env var

Author: rgommers

docs/reference/environment-variables.rst

@@ -59,6 +59,12 @@ Environment variables
    possible for ``meson-python`` to detect this, and it will not set the
    Python wheel platform tag accordingly.
 
+.. envvar:: MESON
+
+   Specifies the ``meson`` executable or script to use. It overrides
+   ``tool.meson-python.meson``. See :ref:`reference-pyproject-settings` for
+   more details.
+
 .. envvar:: MESONPY_EDITABLE_VERBOSE
 
    Setting this environment variable to any value enables directing to the
@@ -68,7 +74,7 @@ Environment variables
 
 .. envvar:: NINJA
 
-   Specify the ninja_ executable to use. It can also be used to select
+   Specifies the ninja_ executable to use. It can also be used to select
    ninja_ alternatives like samurai_.
 
 .. _ninja: https://ninja-build.org

docs/reference/pyproject-settings.rst

@@ -26,6 +26,14 @@ use them and examples.
    ``-Dpython.allow_limited_api=false`` option is passed to ``meson
    setup``.
 
+.. option:: tool.meson-python.meson
+
+   A string specifying the ``meson`` executable or script to use. If it is a
+   path to an existing file with a name ending in ``.py``, it will be invoked
+   as a Python script using the same Python interpreter that is used to run
+   ``meson-python`` itself. It can be overrridden by the :envvar:`MESON`
+   environment variable.
+
 .. option:: tool.meson-python.args.dist
 
    Extra arguments to be passed to the ``meson dist`` command.