moveChunk find parameter needs additional usage info

The "find" parameter in the moveChunk command is misleading, as it sounds like any regular find query could be used here to specify a chunk. This is not the case and needs to be stated more clearly.

In particular, the find parameter must include the shard key as an equality match, it is not possible to include any comparison operators like $gt or $lt.

Doing this will not result in an error, but instead in a different chunk to be moved, as the value will be treated as a document type which compares higher than any numbers or strings.