pgsql: Assorted minor doc/comment fixes.

Assorted minor doc/comment fixes.

Identify pg_replication_origin as a shared catalog in catalogs.sgml,
using the same boilerplate wording used for most other shared catalogs
(and tweak another place where someone had randomly deviated from
that boilerplate).

Make an example in mmgr/README more consistent with surrounding text.

Update an obsolete cross-reference in a comment in storage/block.h.

Zhuo Ql

Discussion: https://postgr.es/m/44296255.1819230.1524889719001@mail.yahoo.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/4094031dd3392b2f49b190a5617d0c247167bcf6

Modified Files
--------------
doc/src/sgml/catalogs.sgml    | 9 ++++++++-
src/backend/utils/mmgr/README | 2 +-
src/include/storage/block.h   | 2 +-
3 files changed, 10 insertions(+), 3 deletions(-)