docs/devel: more documentation on the use of suffixes

Using _qemu is a little confusing. Let's use _compat for these sorts
of things. We should also mention _impl which is another common suffix
in the code base.

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Message-Id: <20220105135009.1584676-25-alex.bennee@linaro.org>

docs/devel/style.rst

@@ -151,6 +151,12 @@ If there are two versions of a function to be called with or without a
 lock held, the function that expects the lock to be already held
 usually uses the suffix ``_locked``.
 
+If a function is a shim designed to deal with compatibility
+workarounds we use the suffix ``_compat``. These are generally not
+called directly and aliased to the plain function name via the
+pre-processor. Another common suffix is ``_impl``; it is used for the
+concrete implementation of a function that will not be called
+directly, but rather through a macro or an inline function.
 
 Block structure
 ===============