docs/devel: Rules on #include in headers

Rules for headers were proposed a long time ago, and generally liked:

    Message-ID: <87h9g8j57d.fsf@blackfin.pond.sub.org>
    https://lists.nongnu.org/archive/html/qemu-devel/2016-03/msg03345.html

Wortk them into docs/devel/style.rst.

Suggested-by: Bernhard Beschow <shentey@gmail.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>
Message-Id: <20221222120813.727830-5-armbru@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Bernhard Beschow <shentey@gmail.com>

docs/devel/style.rst

@@ -293,6 +293,13 @@ that QEMU depends on.
 Do not include "qemu/osdep.h" from header files since the .c file will have
 already included it.
 
+Headers should normally include everything they need beyond osdep.h.
+If exceptions are needed for some reason, they must be documented in
+the header.  If all that's needed from a header is typedefs, consider
+putting those into qemu/typedefs.h instead of including the header.
+
+Cyclic inclusion is forbidden.
+
 C types
 =======