List keybinding for displaying media attachments in the help

[ghost]

To make it possible to discover the M keybinding to show media through normal use, it would be nessesary to add this binding in the help screen

[chiizujin]

The key isn't listed on screen or in the help window but you can press M to open the media in an external program. It tries these programs in this order:

feh, eog, display (ImageMagick)

(I'm using the PyPI version on Linux)

[ghost]

Thanks. Updated title and description to make issue fit more

[chiizujin]

It looks like there's a pull request for this (https://github.com/ihabunek/toot/pull/155) from 2020 with some commentary.