Docs: Permission Group Hold Priority field should include how set values work

[Michele Morgan]

In the settings for Permission Groups, the field Hold Priority defaults to zero, and can be set to a positive or negative number.

It should be documented that the lower the number, the higher the priority. So -1 results in a higher priority than the default of 0.

Including some links to mailing list:

http://georgialibraries.markmail.org/message/4rsqdbmywzuk7awn?q=hold+priority

and irc:

http://irc.evergreen-ils.org/evergreen/2016-10-11#i_271936