Log Message:
-----------
Add "May/Can/Might" section to error message style guidlines, and
"can't" -> "cannot" section.
Modified Files:
--------------
pgsql/doc/src/sgml:
sources.sgml (r2.25 -> r2.26)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/sources.sgml.diff?r1=2.25&r2=2.26)