Fix doc entry for corpus_iterable in train

[svenski]

Fix doc entry for corpus_iterable in train (previously sentences).

[piskvorky]

Thanks. One more little improvement, to clarify the motivation for "streamed input".

[svenski]

Hmm, sorry, closing the commit was the wrong thing to do, re-opening!

[gojomo]

Note that another in-progress PR #2944, in the course of fixing & cleaning up other things, has eliminated the FastText.train() method entirely in favor of simply sharing the implementation from Word2Vec.

I've made sure the Word2Vec.train() comment is similarly fixed in that PR via this commit, so this PR is now superfluous.

Thanks for pointing this out, though, & please keep highlighting/proposing fixes for such errors!

[svenski]

Ah, ok, no worries, thanks for letting me know.