Fix a set of typos and grammar issues across the tree

Fix a set of typos and grammar issues across the tree

This batch is similar to 462fe0ff6215 and addresses a variety of code
style issues, including grammar mistakes, typos, inconsistent variable
names in function declarations, and incorrect function names in comments
and documentation.  These fixes have accumulated on the community
mailing lists since the commit mentioned above.

Notably, Alexander Lakhin previously submitted a patch identifying many
of the trivial typos and grammar issues that had been reported on
pgsql-hackers.  His patch covered a somewhat large portion of the issues
addressed here, though not all of them.

The documentation changes only affect HEAD.