base: Document caveats about Control.Concurrent.Chan
base: Document caveats about Control.Concurrent.Chan

These are implemented using `MVars` which have known caveats. Suggest
the use of `TChan` from the stm library instead.

Test Plan: n/a

Reviewers: austin, hvr, bgamari

Reviewed By: bgamari

Subscribers: thomie

Differential Revision: https://phabricator.haskell.org/D2047

(cherry picked from commit 0f0c1387882be6a19af2cf3e6a005d70a3e84ae7)

GaloisInc/halvm-ghc,GaloisInc/halvm-ghc,GaloisInc/halvm-ghc,GaloisInc/halvm-ghc,GaloisInc/halvm-ghc,GaloisInc/halvm-ghc,GaloisInc/halvm-ghc