main/fish: make util-linux a dep of fish-doc and add a README

It is required to have util-linux to use the help command in any of the builtins fish provides as the internal __fish_print_help function uses the ul program that util-linux provides.

__fish_print_help also returns early if there are no manpages installed which are present in fish-doc so can we can kludingly deal with this by tying the util-linux dependency to the -doc subpackage.

This is not perfect as many users that want fish-doc don't want util-linux but it is better than giving a free util-linux dependency to anyone that installs fish