Assorted typo fixes

Hi folks,

I was playing around with the `typos` tool
(https://github.com/crate-ci/typos), and thought I'd run it on the
posgres repo for fun. After a bit of tweaking to get rid of most false
positives (see separately attached .typos.toml file), it came up with a
useful set of suggestions, some of which I applied verbatim, others
which needed a bit more rewording.

Attached is a series of patches. The first one are what I consider
obvious, unambiguous fixes to code comments. The subsequent ones are
fixes for actual code (variable, function, type names) and docs, one
patch per class of typo. As far as I can tell, none of the code changes
(except the ECPG one, see below) affect anything exported, so this
should not cause any compatibility issues for extensions.

The ECPG change affects the generated C code, but from my reading of the
callers in descriptor.c and ecpg.trailer, any code that would have
caused it to encounter the affected enum value would fail to compile, so
either the case is not possible, or nobody actually uses whatever syntax
is affected (I don't know enough about ECPG to tell without spending far
too much time digging in the code).

- ilmari