bitbake: doc: bitbake-user-manual-ref-variables: document BB_GIT_DEFAULT_DESTSUFFIX

This variable was recently added with 378db0fdd95f ("fetch2/git: allow
overriding default unpack directory with a variable").

(Bitbake rev: a569be0d369cb8b9457beedfeb75b984f751ba5a)

Signed-off-by: Antonin Godard <antonin.godard@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

bitbake/doc/bitbake-user-manual/bitbake-user-manual-ref-variables.rst

@@ -310,6 +310,11 @@ overview of their function and contents.
 
       For example usage, see :term:`BB_GIT_SHALLOW`.
 
+   :term:`BB_GIT_DEFAULT_DESTSUFFIX`
+      The default destination directory where the Git fetcher unpacks the
+      source code. If this variable is not set, the source code is unpacked in a
+      directory named "git".
+
    :term:`BB_GIT_SHALLOW`
       Setting this variable to "1" enables the support for fetching, using and
       generating mirror tarballs of `shallow git repositories <https://riptutorial.com/git/example/4584/shallow-clone>`_.