Extra docs on exports_from_avail.
Extra docs on exports_from_avail.

Signed-off-by: Edward Z. Yang <dbd597f5635f432486c5d365e9bb585b3eaa1853@cs.stanford.edu>

ezyang/ghc,ezyang/ghc,sdiehl/ghc,ezyang/ghc,shlevy/ghc,sdiehl/ghc,shlevy/ghc,shlevy/ghc,sdiehl/ghc,shlevy/ghc,sdiehl/ghc,shlevy/ghc,ezyang/ghc,ezyang/ghc,sdiehl/ghc,ezyang/ghc,shlevy/ghc,shlevy/ghc,ezyang/ghc,sdiehl/ghc,sdiehl/ghc