pgsql: Remove old claim that ExclusiveLock is sometimes taken on system

Remove old claim that ExclusiveLock is sometimes taken on system catalogs.

We used to do that on pg_listener, but pg_listener is no more.

Also add a bit more documentation for ShareRowExclusive mode.

Branch
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master

Details
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http://git.postgresql.org/gitweb?p=postgresql.git;a=commitdiff;h=5042d16d1210d1a10a896a0fcf81230ff4b0e383

Modified Files
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doc/src/sgml/mvcc.sgml | 7 ++++---
1 files changed, 4 insertions(+), 3 deletions(-)