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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REL9_6_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/9e67a9440736be87d4013ce2d23faa131523c07e

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