Update copymode.md: description of `O` key assignment
"only in Rectangular mode" is not correct anymore as at the
end of PR #2150, handling of `O` was changed to also work in
non-Rectangular modes.
* Make alphabet and patterns configurable
* add docs
* Enhance scrollback search to support regex captures so that
searching for eg: `fo(o)` will select the last `o` in `foo`.
refs: https://github.com/wez/wezterm/issues/732
