GHA/checkdocs: re-enable proselint, update setup, fix issues found

- update configuration and invocation.
- install via pip.
- drop a file exception.
- alpha sort proselint settings.
- FILEFORMAT: update text about XML compliance.
- CI job takes 22 seconds total.

Ref: https://github.com/amperser/proselint/releases/tag/v0.16.0

Follow-up to 38bfe1c2aa #15314

Closes #19931

docs/tests/FILEFORMAT.md

@@ -11,10 +11,9 @@ XML. All data for a single test case resides in a single ASCII file. Labels
 mark the beginning and the end of all sections, and each label must be written
 in its own line. Comments are either XML-style (enclosed with `<!--` and
 `-->`) or shell script style (beginning with `#`) and must appear on their own
-lines and not alongside actual test data. Most test data files are
-syntactically-valid XML (a few files are not); lack of support for character
-entities is a big difference but macros like %CR fill that particular role
-here.
+lines and not alongside actual test data. Test data files are syntactically
+valid XML; lack of support for character entities is a big difference but macros
+like %CR fill that particular role here.
 
 Each test case source exists as a file matching the format
 `tests/data/testNUM`, where `NUM` is the unique test number, and must begin