Give the merge copy parameter better documentation

[cpcloud]

See this SO question

The docstring should indicate what is not being copied unnecessarily (Indexes, ndarrays, etc.)

[jreback]

IIRC I think the copy parameter only matters here is its a trivial merge and you actually do want it copied (kind I like a reindex with the same index)

[cpcloud]

yep here's the explanation:

if the index join is the same as both indexes in the join it will return None for the indexers and when _JoinUnit.reindex_block is called copy will only take effect if the indexer is None which means what you said: trivial merge

[cpcloud]

still should say something about that....definitely not what i just wrote but something more informative than what's there

[moaraccounts]

take