Fix documentation for CREATE SEQUENCE IF NOT EXISTS.

The [ IF NOT EXISTS ] was put in wrong place in the syntax.

Pointed out by Marti Raudsepp.
Check for GiST index tuples that don't fit on a page.

The page splitting code would go into infinite recursion if you try to
insert an index tuple that doesn't fit even on an empty page.

Per analysis and suggested fix by Andrew Gierth. Fixes bug #11555, reported
by Bryan Seitz (analysis happened over IRC). Backpatch to all supported
versions.
Still another typo fix for 0709b7ee72e4bc71ad07b7120acd117265ab51d0.

Buildfarm member anole caught this one.
Fix typos in comments.

Etsuro Fujita