Add OSX keystroke to clear RPCConsole

Summary:
Currently only Ctrl-L is mentioned in help, but, (⌘)-L functions on OSX and isn't mentioned.

Backport of Core PR #10362
Completes T431

Co-authored-by: Jason B. Cox <contact@jasonbcox.com>

Test Plan: ninja check

Reviewers: deadalnix, schancel, #bitcoin_abc

Reviewed By: schancel, #bitcoin_abc

Subscribers: teamcity

Differential Revision: https://reviews.bitcoinabc.org/D1931