portal: Document the symbolic names of the flags
portal: Document the symbolic names of the flags

They aren't currently in any installed header file, but that doesn't
mean we can't suggest a name for dependent projects to #define.

Signed-off-by: Simon McVittie <0e1aea601907c1a85c2dc96e5ea37e23c58d2e9b@collabora.com>

flatpak/flatpak,flatpak/flatpak,flatpak/flatpak,flatpak/flatpak,flatpak/flatpak