pgsql: doc: clarify syntax notation, particularly parentheses

doc: clarify syntax notation, particularly parentheses

Also move TCL syntax to the PL/tcl section.

Reported-by: davs2rt@gmail.com

Discussion: /messages/by-id/164308146320.12460.3590769444508751574@wrigleys.postgresql.org

Backpatch-through: 10

Branch
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REL_10_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/03739f23ff9911aea7d58c9cc0b58109c8ea95a6

Modified Files
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doc/src/sgml/notation.sgml | 6 +++---
doc/src/sgml/pltcl.sgml | 6 ++++--
2 files changed, 7 insertions(+), 5 deletions(-)