doc/develop/codingstyle.rst: Clarify include section

Rework the section about includes slightly. We should not be using
common.h anywhere, so remove that from examples and ask people to send
patches removing it when found. Doing this also means we need to reword
other parts of this section. Be clearer about using alphabetical
ordering.

Signed-off-by: Tom Rini <trini@konsulko.com>
Reviewed-by: Igor Opaniuk <igor.opaniuk@gmail.com>