x11-wm/hyprland: document plugins

Unlike Hyprland itself which is tricky to build outside of ports
plugins can be installed/enabled via hyprpm. If some plugins fail to
build the cause is usually Clang/libc++ vs. GCC/libstdc++, so have
nothing to do with FreeBSD and should be reported upstream.

(cherry picked from commit 83486fcc5ad9f2da11ccd5ef5be9c43b13aacfbb)