docs: orderByPriority shouldn't be promoted

[WhatsThatItsPat]

Version info

Angular:
4.1.2

Firebase:
4.0.0

AngularFire:
4.0.0-rc.0

Other (e.g. Ionic/Cordova, Node, browser, operating system):
N/A

How to reproduce these conditions

In Querying lists of the Developer Guide, orderByPriority is suggested as an equal to other orderBy* methods while it is no longer recommended.

Debug output

N/A

Expected behavior

N/A

Actual behavior

N/A

[jamesdaniels]

Merged.