manual: contributing: don't recommend leaving out changelog from patches

Currently, the manual states that if you send a series and include a
changelog in the introduction mail (0 of x), it is not necessary to include
a changelog on the individual patches.
This is not always convenient: for large patch series it requires going back
to the introduction mail when opening each patch, to understand what
changed.
This patch rewords the paragraph accordingly.

Signed-off-by: Thomas De Schampheleire <thomas.de.schampheleire@gmail.com>
Acked-by: Samuel Martin <s.martin49@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>

docs/manual/contribute.txt

@@ -228,8 +228,8 @@ Patch revision changelog
 When improvements are requested, the new revision of each commit
 should include a changelog of the modifications between each
 submission. Note that when your patch series is introduced by a cover
-letter, the changelog may be added in the cover letter rather than in
-the individual commits.
+letter, an overall changelog may be added to the cover letter in
+addition to the changelog in the individual commits.
 
 When added to the individual commits, this changelog is added when
 editing the commit message.  Below the +Signed-off-by+ section, add