I've attached a patch to get symbol documentation working in the keymap buffer.
Note that I've rebound mouse-3 as the insert command in deference to platforms that don't have a convenient way to emulate the middle mouse button.
I've also bound ? to bring up help on the symbol under point.
Help is only available for the symbols on the APL side of each key. The keymap buffer won't offer help on symbols such as ravel, compress and expand. This is an existing limitation of the help buffer code.
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"The secret to creativity is knowing how to hide your sources."