CODING_STYLE: Allow single-sentence comments without full stops

CODING_STYLE: Allow single-sentence comments without full stops

One of the common ways in which contributors trip up over the
CODING_STYLE guides is by not putting a full stop at the end of a
comment when there is only a single sentence.  Calling these out is a
waste of everybody's time: The full stop at the end of a comment with
a single sentence (or a single phrase) adds absolutely nothing to the
legibility of the code.

Modify CODING_STYLE to allow comments with a single sentence or
sentence fragment to either have a full stop or not, while making it
clear that comments with multiple sentences must have a full stop at
the end of each sentence.

Signed-off-by: George Dunlap <george.dunlap@citrix.com>
Acked-by: Ian Jackson <ian.jackson@citrix.com>