Avoid memory leak from incorrect choice of how to free a StringInfo
(resetStringInfo doesn't do it).  Now that pg_split_opts doesn't scribble
on the optstr, mark that as "const" for clarity.  Attach the commentary in
protocol.sgml to the right place, and add documentation about the
user-visible effects of this change on postgres' -o option and libpq's
PGOPTIONS option.