Please document DisableAvatarsThreshold in the manpage

[maxyz]

Hi,

This is related to the bug reported to the Debian bug tracker in: https://bugs.debian.org/856195

After upgrading to 0.14.0 users of systems that have more than 7 users stopped seeing their configured avatars due to the new option DisableAvatarsThreshold, this option isn't documented in the sddm.conf manpage, which reduces the visibility of the change.

Also, this option uses the configuration reader isDefault which breaks the expectancy that installing the generated sddm.conf doesn't change sddm's behavior. Please consider using a tristate for EnableAvatars: true, false, threshold (default) instead.

Happy hacking,