Document need for `<lt>`/`<gt>` in macro keybinds

[david-crespo]

I just learned on matrix that in order to put </> as a literal in a macro keybind, you have to use <lt>/<gt> (<<> might work for < but <>> does not work for >). This should be documented in the key remapping docs. Use the word "angle bracket" so it can be found more easily. I'll try to put up a PR later about it.