pgsql: Update README, we don't do post-recovery cleanup actions anymore

Update README, we don't do post-recovery cleanup actions anymore.

transam/README explained how B-tree incomplete splits were tracked and
fixed after recovery, as an example of handling complex actions that need
multiple WAL records, but that's not how it works anymore. Explain the new
paradigm.

Branch
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master

Details
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http://git.postgresql.org/pg/commitdiff/a3655dd4a5cee3917a7d1766e07e36013e7e8835

Modified Files
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src/backend/access/transam/README |   25 +++++++++++++++----------
1 file changed, 15 insertions(+), 10 deletions(-)