Document another unsafeDupablePerformIO limitation.
Document another unsafeDupablePerformIO limitation.

Signed-off-by: Austin Seipp <48c905ac3851f382b914247d97dd93273d5ff555@pobox.com>

ezyang/ghc,mettekou/ghc,GaloisInc/halvm-ghc,AlexanderPankiv/ghc,ml9951/ghc,acowley/ghc,holzensp/ghc,da-x/ghc,sgillespie/ghc,tibbe/ghc,anton-dessiatov/ghc,christiaanb/ghc,mettekou/ghc,tjakway/ghcjvm,snoyberg/ghc,mettekou/ghc,gridaphobe/ghc,jstolarek/ghc,vikraman/ghc,ghc-android/ghc,tibbe/ghc,ml9951/ghc,christiaanb/ghc,TomMD/ghc,gcampax/ghc,holzensp/ghc,fmthoma/ghc,nushio3/ghc,gcampax/ghc,shlevy/ghc,nkaretnikov/ghc,acowley/ghc,frantisekfarka/ghc-dsi,olsner/ghc,siddhanathan/ghc,sdiehl/ghc,gcampax/ghc,gridaphobe/ghc,ghc-android/ghc,fmthoma/ghc,elieux/ghc,bitemyapp/ghc,tibbe/ghc,AlexanderPankiv/ghc,frantisekfarka/ghc-dsi,ryantm/ghc,anton-dessiatov/ghc,olsner/ghc,TomMD/ghc,green-haskell/ghc,nkaretnikov/ghc,anton-dessiatov/ghc,green-haskell/ghc,forked-upstream-packages-for-ghcjs/ghc,GaloisInc/halvm-ghc,tjakway/ghcjvm,bitemyapp/ghc,tjakway/ghcjvm,lukexi/ghc,GaloisInc/halvm-ghc,mcschroeder/ghc,sdiehl/ghc,siddhanathan/ghc,sdiehl/ghc,sgillespie/ghc,nushio3/ghc,vikraman/ghc,gridaphobe/ghc,oldmanmike/ghc,da-x/ghc,ghc-android/ghc,christiaanb/ghc,fmthoma/ghc,mcschroeder/ghc,nushio3/ghc,snoyberg/ghc,forked-upstream-packages-for-ghcjs/ghc,spacekitteh/smcghc,snoyberg/ghc,urbanslug/ghc,ryantm/ghc,tibbe/ghc,GaloisInc/halvm-ghc,mettekou/ghc,ryantm/ghc,GaloisInc/halvm-ghc,urbanslug/ghc,AlexanderPankiv/ghc,vikraman/ghc,GaloisInc/halvm-ghc,holzensp/ghc,christiaanb/ghc,nushio3/ghc,spacekitteh/smcghc,GaloisInc/halvm-ghc,christiaanb/ghc,olsner/ghc,ezyang/ghc,nushio3/ghc,bitemyapp/ghc,nathyong/microghc-ghc,oldmanmike/ghc,lukexi/ghc,tjakway/ghcjvm,AlexanderPankiv/ghc,olsner/ghc,nathyong/microghc-ghc,spacekitteh/smcghc,shlevy/ghc,vTurbine/ghc,oldmanmike/ghc,green-haskell/ghc,tibbe/ghc,spacekitteh/smcghc,urbanslug/ghc,acowley/ghc,mfine/ghc,spacekitteh/smcghc,vikraman/ghc,tjakway/ghcjvm,shlevy/ghc,vTurbine/ghc,mcschroeder/ghc,ml9951/ghc,urbanslug/ghc,vikraman/ghc,vTurbine/ghc,sdiehl/ghc,frantisekfarka/ghc-dsi,elieux/ghc,olsner/ghc,ezyang/ghc,olsner/ghc,nathyong/microghc-ghc,ml9951/ghc,elieux/ghc,elieux/ghc,gcampax/ghc,mcschroeder/ghc,snoyberg/ghc,jstolarek/ghc,elieux/ghc,wxwxwwxxx/ghc,oldmanmike/ghc,gridaphobe/ghc,sgillespie/ghc,christiaanb/ghc,sgillespie/ghc,wxwxwwxxx/ghc,shlevy/ghc,ml9951/ghc,jstolarek/ghc,frantisekfarka/ghc-dsi,sgillespie/ghc,mfine/ghc,TomMD/ghc,lukexi/ghc,oldmanmike/ghc,shlevy/ghc,siddhanathan/ghc,vTurbine/ghc,nathyong/microghc-ghc,acowley/ghc,oldmanmike/ghc,ryantm/ghc,vTurbine/ghc,green-haskell/ghc,wxwxwwxxx/ghc,elieux/ghc,TomMD/ghc,nushio3/ghc,nkaretnikov/ghc,acowley/ghc,TomMD/ghc,ghc-android/ghc,nathyong/microghc-ghc,da-x/ghc,forked-upstream-packages-for-ghcjs/ghc,wxwxwwxxx/ghc,gridaphobe/ghc,gcampax/ghc,acowley/ghc,ml9951/ghc,ezyang/ghc,forked-upstream-packages-for-ghcjs/ghc,sdiehl/ghc,anton-dessiatov/ghc,nkaretnikov/ghc,mfine/ghc,mcschroeder/ghc,da-x/ghc,vikraman/ghc,tjakway/ghcjvm,ml9951/ghc,sgillespie/ghc,anton-dessiatov/ghc,green-haskell/ghc,nathyong/microghc-ghc,da-x/ghc,siddhanathan/ghc,mcschroeder/ghc,AlexanderPankiv/ghc,ml9951/ghc,snoyberg/ghc,wxwxwwxxx/ghc,gridaphobe/ghc,elieux/ghc,gcampax/ghc,anton-dessiatov/ghc,da-x/ghc,frantisekfarka/ghc-dsi,AlexanderPankiv/ghc,bitemyapp/ghc,fmthoma/ghc,ghc-android/ghc,nkaretnikov/ghc,fmthoma/ghc,ezyang/ghc,jstolarek/ghc,ghc-android/ghc,jstolarek/ghc,oldmanmike/ghc,snoyberg/ghc,tjakway/ghcjvm,forked-upstream-packages-for-ghcjs/ghc,olsner/ghc,ryantm/ghc,wxwxwwxxx/ghc,mettekou/ghc,sdiehl/ghc,shlevy/ghc,holzensp/ghc,nathyong/microghc-ghc,TomMD/ghc,christiaanb/ghc,ezyang/ghc,urbanslug/ghc,sgillespie/ghc,nkaretnikov/ghc,lukexi/ghc,ghc-android/ghc,mfine/ghc,urbanslug/ghc,bitemyapp/ghc,anton-dessiatov/ghc,da-x/ghc,mfine/ghc,mcschroeder/ghc,snoyberg/ghc,fmthoma/ghc,urbanslug/ghc,siddhanathan/ghc,nkaretnikov/ghc,gcampax/ghc,vTurbine/ghc,vikraman/ghc,sdiehl/ghc,forked-upstream-packages-for-ghcjs/ghc,TomMD/ghc,gridaphobe/ghc,siddhanathan/ghc,wxwxwwxxx/ghc,ezyang/ghc,vTurbine/ghc,mettekou/ghc,forked-upstream-packages-for-ghcjs/ghc,mfine/ghc,lukexi/ghc,siddhanathan/ghc,holzensp/ghc,acowley/ghc,fmthoma/ghc,mfine/ghc,nushio3/ghc,shlevy/ghc,AlexanderPankiv/ghc,mettekou/ghc