Comment clarification/typo fixes about extensionPoints, across six files (#14811)
Summary of the issue:
While working on #14810, I found several places where comments about the core extensionPoints could be made clearer, or had typos/capitalization errors/questionable grammar.
Description of user facing changes
None.
Description of development approach
I fixed them as I went along, and scooped them all into their own branch. Here is the result.
I changed no code in this, and nothing structural has been altered. That is, where #: style comments were used in some places, and docstring style comments in others, I left them that way. I'm not actually sure what we prefer now. I wasn't trying to lint anything, just fix language.