docs/manual: standardize a bit more the formatting of commit titles

docs/manual: standardize a bit more the formatting of commit titles

Currently, our commit titles are not very well standardized, and it
would be great to standardize them a little bit more. A number of
people use "<pkg>: " as prefix, others use "package/<pkg>: ". Some
people start the rest of the commit title (after the prefix) with an
upper-case letter, some with a lower-case letter.

In an attempt to standardize this, this commit updates the manual with
some examples of good commit titles.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Reviewed-by: Carlos Santos <casantos@datacom.com.br>
Acked-by: Peter Korsgaard <peter@korsgaard.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>