Update docs for LABEL PRIORITY

[dmorissette]

This ticket is to track docs updates for the new LABEL PRIORITY feature added in ticket #1619.

See also RFC 27: ​http://mapserver.gis.umn.edu/development/rfc/ms-rfc-27

[dmorissette]

Done. The following has been added to the label object docs in the mapfile reference:

PRIORITY [integer]|[item_name]
     The priority parameter (added in v5.0) takes an integer value between 1 
     (lowest) and 10 (highest). The default value is 1. It is also possible
     to bind the priority to an attribute (item_name) using square brackets 
     around the [item_name]. e.g. "PRIORITY [someattribute]"

     Labels are stored in the label cache and rendered in order of priority, 
     with the highest priority levels rendered first.
     Specifying an out of range PRIORITY value inside a map file will result in
     a parsing error. An out of range value set via MapScript or coming from a 
     shape attribute will be clamped to the min/max values at rendering time.
     There is no expected impact on performance for using label priorities.