Fixing various typos in comments and docs

This patch fixes various typos I've found, most of them from recent
commits. I think none should be controversial except perhaps

--- a/doc/src/sgml/ref/pgupgrade.sgml
+++ b/doc/src/sgml/ref/pgupgrade.sgml
@@ -801,7 +801,7 @@ psql --username=postgres --file=script.sql postgres
</para>

<para>
- Because not all statistics are not transferred by
+ Because not all statistics are transferred by
<command>pg_upgrade</command>, you will be instructed to run a
command to
regenerate that information at the end of the upgrade. You might
need to
set connection parameters to match your new cluster.

Separately from this, I have been working on some tooling to flag typos in
new commits. Is that something we'd ever want to automate?

Regards,
Jacob