doc: clarify what type of changes are acceptable at commit time

As a rule of thumb, don't change patches while committing.

v2: Added note about conflicts (Daniel)

Cc: Imre Deak <imre.deak@intel.com>
Acked-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>