docs: recommend orderByChild in favor of orderByPriority

[WhatsThatItsPat]

Checklist

• Issue number for this PR: docs: orderByPriority shouldn't be promoted #1005
• Docs included?: yes
• Test units included?: no, N/A
• e2e tests included?: no, N/A
• In a clean directory, npm install, npm run build, and npm test run successfully? no, N/A

Description

Update to Querying lists docs to recommend orderByChild in favor of orderByPriority.

Code sample

N/A