runtime(doc): Improve :help builtin-function-list table formatting

- Start each field at the correct tabstop - on a new line if required.
- Use "Type1/Type2" for return types rather than "Type1 or Type2",
  matching the dominant style.
- Convert hyperlinked Ex commands to command markup, `:cmd`.
- Wrap overlong lines.

closes: #18438

Signed-off-by: Doug Kearns <dougkearns@gmail.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>

Author: dkearns