pgsql: doc: clarify when row-level locks are released

doc: clarify when row-level locks are released

They are released just like table-level locks. Also clean up wording.

Reported-by: me@sillymon.ch

Discussion: /messages/by-id/158074944048.1095.4309647363871637715@wrigleys.postgresql.org

Backpatch-through: 9.5

Branch
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REL9_6_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/6e727a13cbc189e5974c3e8f6fa3eff9f87d1417

Modified Files
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doc/src/sgml/mvcc.sgml | 9 ++++++---
1 file changed, 6 insertions(+), 3 deletions(-)