The elvdocs still use the old format (#elvdoc:fn or #elvdoc:var) for now, but
will be changed to "fn" and "var" forms soon.
Also remove the accidentally committed cmd/mvelvdoc. It has been used to perform
the conversion automatically but is not supposed to be committed.
* Allow $nil for Callable options
Related #1570
* Add a `&key` option to the `order` command.
Resolves#1570
Co-authored-by: Qi Xiao <xiaq@users.noreply.github.com>
- Revise the introduction section.
- Link to the notes on commands taking value inputs from all such commands.
- Revise the doc on the "all" command.
- Revise references to the builtin module from the language reference.