docs(commit-style): update commit style guide

This patch updates commit-style.rst to reflect the rules enforced by
checkcommitmsg.py. A maximum width of 72 characters is now enforced for
commit messages.

Signed-off-by: Chuyue Luo <Chuyue.Luo@arm.com>
Change-Id: I88bbf345d53df050b1fa2680ff0ec1a120b34697
diff --git a/docs/process/commit-style.rst b/docs/process/commit-style.rst
index 5af5fab..b91ed90 100644
--- a/docs/process/commit-style.rst
+++ b/docs/process/commit-style.rst
@@ -83,6 +83,8 @@
     Change-Id: ...
     Signed-off-by: ...
 
+In addition, the width of the commit message must be no more than 72 characters.
+
 .. _mandated-trailers:
 
 Mandated Trailers