pgsql: Clarify documentation of RESET ROLE

Clarify documentation of RESET ROLE

Command-line options, or previous "ALTER (ROLE|DATABASE) ...
SET ROLE ..." commands, can change the value of the default role
for a session. In the presence of one of these, RESET ROLE will
change the current user identifier to the default role rather
than the session user identifier. Fix the documentation to
reflect this reality. Backpatch to all supported versions.

Author: Nathan Bossart
Reviewed-By: Laurenz Albe, David G. Johnston, Joe Conway
Reported by: Nathan Bossart
Discussion: https://postgr.es/m/flat/925134DB-8212-4F60-8AB1-B1231D750CB4%40amazon.com
Backpatch-through: 9.6

Branch
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REL_13_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/4a1b95fcf4710c9578750d1d9ab4672eab3a5a82

Modified Files
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doc/src/sgml/ref/set_role.sgml | 13 ++++++++++---
1 file changed, 10 insertions(+), 3 deletions(-)