add paragraph about meta_queries and NOT_EXISTS

[moraleida]

Description of the Change

Fixes #355

Adds a paragraph discouraging the use of "EXISTS" or "NOT_EXISTS" as comparison methods to query for post meta. Please see full description as examples in the Issue.

Checklist:

• [x] I have read the CONTRIBUTING document.
• [x] My code follows the code style of this project.
• [x] My change requires a change to the documentation.
• [x] I have updated the documentation accordingly.
• [ ] I have added tests to cover my change.
• [ ] All new and existing tests passed.

Applicable Issues

355

Changelog Entry

[jeffpaul]

@bengreeley would appreciate review from you on this, thanks!