Add a help-describe for evil-ex-commands

[gilch]

Unless I've completely missed something important (possible), Evil's ex commands are not discoverable enough. They are not the same as Vim's (Vim has a lot more for one thing), so even experienced Vim users don't know what's there.

I'm proposing adding a help-describe evil-ex-command function. It would work in helm like SPC h d f does now. Possible mnemonics are SPC h d x, SPC h d e, SPC h d :. Alternatively, if possible, just add the evil-ex-commands to the existing SPC h d f search.

[github-actions]

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