Docstrings for custom keymappings beneath a mode

[ElDifinitivo]

When defining new keymappings under an existing mode, the docstring will default to the description of the command, with each new keymap listed as a new entry in the pop-up.

However, when one of the new maps consist of multiple commands (e.g. m = { l = ["extend_to_line_bounds", "trim_selections"] }), the docstring will default to [Multiple commands], which gives the user no indication as to what the keymap will do. There should be an override which would allow the user to set a custom description for the keymap. The default Multiple commands should probably remain as i+ commands could get increasingly unwieldy if it defaulted to the description of each individual command.

[the-mikedavis]

Duplicate of #3130