This commit adds a `documentation` face to the builtin themes, used
to highlight common documentation syntaxes:
/**
* JavaDoc
*/
/*!
* QtDoc
*/
/// Inline documentation
## Inline documentation
The face is only an alias to the `comment` one for now.
Closes#1944
set-face now takes a scope argument, and faces can be overridden on
a buffer or window basis.
colorscheme apply on global scope, which should be good enough for
now.
Fixes#1411
