doc:  add ALTER TABLE lock level item as major 9.4 item

Report by Simon Riggs
Fix typos in comments.
Fix typos in comments.
Fix typo in comment.

Erik Rijkers
Remove unnecessary cleanup code.

This is all inside a block guarded by op == DSM_OP_ATTACH, so it can
never be the case that op == DSM_OP_CREATE.

Reported by Coverity.
Fix misc typos in comments.
doc:  link/caps fixes for 9.4 release notes

Report by Tomonari Katsumata
Silence a couple of spurious valgrind warnings in inval.c.

Define padding bytes in SharedInvalidationMessage structs to be
defined. Otherwise the sinvaladt.c ringbuffer, which is accessed by
multiple processes, will cause spurious valgrind warnings about
undefined memory being used. That's because valgrind remembers the
undefined bytes from the last local process's store, not realizing
that another process has written since, filling the previously
uninitialized bytes.