Document disassociating YZ index with bucket.

[lukebakken]

From the mailing list:

An index may not be deleted if it has any buckets associated with it. The 'search_index' property (not 'search-index') must be changed to either a different index or the sentinel value _dont_index_

[coderoshi]

I'd put a "deleting an index" section under "Using Search" for now

[lucperkins]

#911

[lucperkins]

Addressed in PR #911