pgsql: doc: Update note about source code formatting

doc: Update note about source code formatting

Update the note about why not to use // comments, even though it's now
technically supported.

The note about variable declarations was dropped here because it's
addressed more properly later in the chapter.

Discussion: https://www.postgresql.org/message-id/flat/156924954640.1117.6309209869705522549%40wrigleys.postgresql.org

Branch
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REL_12_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/b4d615aaa1ecedcf1e2ca415cd2893d1f18aeeca

Modified Files
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doc/src/sgml/sources.sgml | 6 +++---
1 file changed, 3 insertions(+), 3 deletions(-)