pgsql: Update code comments about peer authenticaton

Update code comments about peer authenticaton

For historical reasons, the functions for peer authentication were
grouped under ident authentication.  But they are really completely
separate, so give them their own section headings.

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/5cc1e64fb6f59aa8621caae03be36400807e3575

Modified Files
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src/backend/libpq/auth.c | 11 +++++++++++
1 file changed, 11 insertions(+)