pgsql: docs: Consolidate into new "WAL for Extensions" chapter.

docs: Consolidate into new "WAL for Extensions" chapter.

Previously, we had consecutive, very short chapters called "Generic
WAL" and "Custom WAL Resource Managers," explaining different approaches
to the same problem. Merge them into a single chapter. Explain most
of the differences between the approaches in the chapter's
introductory text, rather than in the individual sections.

Discussion: http://postgr.es/m/46ac50c1-6b2a-404f-a683-b67af6ab56e9@eisentraut.org

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/09d9800e52822ed20d69d98aaa39d3fb10769c44

Modified Files
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doc/src/sgml/custom-rmgr.sgml | 11 ++++-------
doc/src/sgml/filelist.sgml | 1 +
doc/src/sgml/generic-wal.sgml | 14 +++-----------
doc/src/sgml/postgres.sgml | 3 +--
doc/src/sgml/wal-for-extensions.sgml | 32 ++++++++++++++++++++++++++++++++
5 files changed, 41 insertions(+), 20 deletions(-)