Docs fixups

I read the docs and felt some things could be improved.

The first 3 commits should be pretty non-controversial (I hope!).

After those, I split the input plugin docs into their own pages. This may be somewhat controversial. The end state of this PR is that input plugin docs are inconsistent from the rest of the plugin docs. If my approach for input docs is approved, I can give similar treatment to the docs for other plugin types.