pgsql: Get rid of GenericXLogUnregister().

Get rid of GenericXLogUnregister().

This routine is unsafe as implemented, because it invalidates the page
image pointers returned by previous GenericXLogRegister() calls.

Rather than complicate the API or the implementation to avoid that,
let's just get rid of it; the use-case for having it seems much
too thin to justify a lot of work here.

While at it, do some wordsmithing on the SGML docs for generic WAL.

Branch
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master

Details
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http://git.postgresql.org/pg/commitdiff/08e785436f84f8824149a2182b0cb9ce2c28e31d

Modified Files
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doc/src/sgml/generic-wal.sgml | 121 +++++++++++++++++-------------
src/backend/access/transam/generic_xlog.c | 29 -------
src/include/access/generic_xlog.h | 3 +-
3 files changed, 69 insertions(+), 84 deletions(-)