x11-fonts/font-awesome: Updated pkg-descr to reflect the fact that only desktop fonts are installed.

The change is originally requested by Adam Weinberger <adamw@FreeBSD.org>.

PR: 223616
Approved by: tcberner (mentor), Dmitri Goutnik <dg@syrec.org> (maintainer)
Differential Revision: https://reviews.freebsd.org/D13054