pgsql: Doc: fix confusion about LEAKPROOF in syntax summaries.

Doc: fix confusion about LEAKPROOF in syntax summaries.

The syntax summaries for CREATE FUNCTION and allied commands
made it look like LEAKPROOF is an alternative to
IMMUTABLE/STABLE/VOLATILE, when of course it is an orthogonal
option.  Improve that.

Per gripe from aazamrafeeque0.  Thanks to David Johnston for
suggestions.

Discussion: https://postgr.es/m/162444349581.694.5818572718530259025@wrigleys.postgresql.org

Branch
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REL_11_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/5c22cf0e73397dfb83a91b14f10ca610dd83cff3

Modified Files
--------------
doc/src/sgml/ref/alter_function.sgml  | 3 ++-
doc/src/sgml/ref/alter_routine.sgml   | 3 ++-
doc/src/sgml/ref/create_function.sgml | 7 ++++---
3 files changed, 8 insertions(+), 5 deletions(-)