Removed info about forcing disaggregation from image driver docs.

[tbohn]

• [x] closes #830
• [ ] tests passed
• [ ] new tests added
• [ ] science test figures
• [ ] ran uncrustify prior to final commit
• [x] ReleaseNotes entry

[tbohn]

Added release note entry for PR #823 (unrelated to forcing disagg docs, but related to this under the umbrella of bringing documentation up to date).