[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>

scripts/gen_vimdoc.py

@@ -2,9 +2,7 @@
 """Generates Nvim :help docs from C/Lua docstrings, using Doxygen.
 
 Also generates *.mpack files. To inspect the *.mpack structure:
-
-    :new | put=v:lua.vim.inspect(msgpackparse(readfile('runtime/doc/api.mpack')))
-
+    :new | put=v:lua.vim.inspect(v:lua.vim.mpack.unpack(readfile('runtime/doc/api.mpack','B')))
 
 Flow:
     main
@@ -287,7 +285,7 @@ annotation_map = {
     'FUNC_API_FAST': '|api-fast|',
     'FUNC_API_CHECK_TEXTLOCK': 'not allowed when |textlock| is active',
     'FUNC_API_REMOTE_ONLY': '|RPC| only',
-    'FUNC_API_LUA_ONLY': '|vim.api| only',
+    'FUNC_API_LUA_ONLY': 'Lua |vim.api| only',
 }