[FileCheck] Fix up -dump-input* docs

In FileCheck.rst, add `-dump-input-context` and `-dump-input-filter`,
and fix some `-dump-input` documentation.

In `FileCheck -help`, `cl::value_desc("kind")` is being ignored for
`-dump-input-filter`, so just drop it.

Extend `-dump-input=help` to mention FILECHECK_OPTS.