docs: lsp, misc

- Problem: It's not clear for new plugin developers that `:help` uses
  a help-tags file for searching the docs, generated by `:helptags`.
  - Solution: Hint to the |:helptags| docs for regenerating the tags
    file for their freshly written documentation.

Co-authored-by: Yochem van Rosmalen <git@yochem.nl>

src/nvim/api/extmark.c

@@ -1033,8 +1033,7 @@ void nvim_buf_clear_namespace(Buffer buffer, Integer ns_id, Integer line_start,
 ///               ```
 ///                 ["win", winid, bufnr, toprow, botrow]
 ///               ```
-///             - on_line: called for each buffer line being redrawn.
-///                 (The interaction with fold lines is subject to change)
+///             - on_line: (deprecated, use on_range instead)
 ///               ```
 ///                 ["line", winid, bufnr, row]
 ///               ```