docs/devel: make a statement about includes

While not explicitly disallowing header macro abuse (because that
would make us hypocrites) lets at least address some things to think
about.

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Juan Quintela <quintela@redhat.com>
Message-Id: <20230424092249.58552-17-alex.bennee@linaro.org>

docs/devel/style.rst

@@ -300,6 +300,20 @@ putting those into qemu/typedefs.h instead of including the header.
 
 Cyclic inclusion is forbidden.
 
+Generative Includes
+-------------------
+
+QEMU makes fairly extensive use of the macro pre-processor to
+instantiate multiple similar functions. While such abuse of the macro
+processor isn't discouraged it can make debugging and code navigation
+harder. You should consider carefully if the same effect can be
+achieved by making it easy for the compiler to constant fold or using
+python scripting to generate grep friendly code.
+
+If you do use template header files they should be named with the
+``.c.inc`` or ``.h.inc`` suffix to make it clear they are being
+included for expansion.
+
 C types
 =======