doc: contribution guidelines: Clarify use of uncrustify

A few new contributors, after reading the uncrustify section
of the contribution guidelines, have decided to run
existing Zephyr files thru uncrustify, and include in commits
with minor fixes lots of styles changes.
This is something we do not want to encourage.

To avoid this, modify a bit the uncrustify section to
discourage people from doing just that.

Signed-off-by: Alberto Escolar Piedras <alpi@oticon.com>