[COMMITTERS] pgsql: Doc: explain dollar quoting in the intro part of the pl/pgsqlch

Doc: explain dollar quoting in the intro part of the pl/pgsql chapter.

We're throwing people into the guts of the syntax with not much context;
let's back up one step and point out that this goes inside a literal in
a CREATE FUNCTION command.  Per suggestion from Kurt Kartaltepe.

Discussion: https://postgr.es/m/CACawnnyWAmH+au8nfZhLiFfWKjXy4d0kY+eZWfcxPRnjVfaa_Q@mail.gmail.com

Branch
------
REL9_2_STABLE

Details
-------
https://git.postgresql.org/pg/commitdiff/2d08d036eae3f50bcda98113a322020479264821

Modified Files
--------------
doc/src/sgml/plpgsql.sgml | 20 +++++++++++++++++++-
1 file changed, 19 insertions(+), 1 deletion(-)