[conduit-combinators] Add warnings about some consumers

[neongreen]

I spent half an hour yesterday trying to understand why a dropWhileC in a pipeline doesn't work, so here are some warnings so that others won't make the same mistake.

 -- | Ignore a certain number of values in the stream.
 --
+-- Note: since this function doesn't produce anything, you probably want to
+-- use it with ('>>') instead of directly plugging it into a pipeline:
+--
+-- >>> runConduit $ yieldMany [1..5] .| drop 2 .| sinkList
+-- []
+-- >>> runConduit $ yieldMany [1..5] .| (drop 2 >> sinkList)
+-- [3,4,5]
+--
 -- Since 1.0.0
 drop :: Monad m
      => Int
      -> Consumer a m ()
INLINE_RULE(drop, n, CL.drop n)

[snoyberg]

Looks like the issue brought up in this blog post: https://www.snoyman.com/blog/2017/05/worst-function-in-conduit. Definitely good to have it explained in the docs like this, thanks! One issue: the code in question is shortly going to be moved over to the conduit package instead as part of the 1.3.0 release. Could you instead send this PR against the conduit repo, e.g. the https://github.com/snoyberg/conduit/blob/master/conduit/src/Data/Conduit/Combinators.hs file?

[neongreen]

Done: https://github.com/snoyberg/conduit/pull/348