[Backport release-0.8] doc (#20616)

docs: various

- increase python line-length limit from 88 => 100.
- gen_help_html: fix bug in "tag" case (tbl_count => tbl_contains)

ref #15632
fix #18215
fix #18479
fix #20527
fix #20532

Co-authored-by: Ben Weedon <ben@weedon.email>
(cherry picked from commit f37cd2bb15)

Co-authored-by: Justin M. Keyes <justinkz@gmail.com>

test/functional/helpers.lua

@@ -275,7 +275,6 @@ function module.command(cmd)
   module.request('nvim_command', cmd)
 end
 
-
 -- Use for commands which expect nvim to quit.
 -- The first argument can also be a timeout.
 function module.expect_exit(fn_or_timeout, ...)