[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
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REL9_3_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/a64726e7a53482fca80860cd0ea5038f465f7fa1

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