Consider noshowmode during cmdheight check

Instead of increasing `cmdheight`, you can set `noshowmode` and see echodoc's information.
This pull-request includes:
- Added `&showmode` condition in `cmdheight` check. `cmdheight` cannot be 0, so this is enough
- Increasing `cmdheight` with error message - otherwise user won't see the error because it disappears quickly
- Adding `noshowmode` option in documentation
- Small typo fix in \*echodoc-introduction\*