9.3 release notes:  update for current commits

Also, remove broken link in release.sgml.
Improve SPI documentation about null-flags arrays.

Clarify the description of nulls[] arguments, and use the same wording
for all SPI functions with this type of argument.  Per gripe from Yuriy
Rusinov.
9.3 release notes:  add markup for text

Still need to add links for new features.
9.3 release notes:  Add links to SGML sections
9.3 release notes:  Add markup for pg_backup_start_time()
9.3 release notes:  move compatibility items into their own section