pgsql: Adjust comments of CheckRelationTableSpaceMove() and SetRelation

Adjust comments of CheckRelationTableSpaceMove() and SetRelationTableSpace()

4c9c359, that introduced those two functions, has been overoptimistic on
the point that only ShareUpdateExclusiveLock would be required when
moving a relation to a new tablespace.  AccessExclusiveLock is a
requirement, but ShareUpdateExclusiveLock may be used under specific
conditions like REINDEX CONCURRENTLY where waits on past transactions
make the operation safe even with a lower-level lock.  The current code
does only the former, so update the existing comments to reflect that.

Once a REINDEX (TABLESPACE) is introduced, those comments would require
an extra refresh to mention their new use case.

While on it, fix an incorrect variable name.

Per discussion with Álvaro Herrera.

Discussion: https://postgr.es/m/20210127140741.GA14174@alvherre.pgsql

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/24843297a96d7be16cc3f4b090aacfc6e5e6839e

Modified Files
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src/backend/commands/tablecmds.c | 13 ++++++-------
1 file changed, 6 insertions(+), 7 deletions(-)