7.10.1 documenation is incorrect wrt supported llvm version

As documented in #9929 (closed) ghc 7.10.1 only supports llvm 3.5.

This should be documented in the release notes section of the GHC Users's Guide, Version 7.10.1.

In addition, the following in the Known bugs section needs to be changed to mention this also:

GHC’s LLVM backend is currently incompatible with LLVM 3.4 (issue #9929 (closed)).

The following in section 4.11.2 should also be changed from:

Currently LLVM 2.8 and later are supported.

to

Only LLVM 3.5 is supported.

from

Secondly, if you are running Mac OS X with LLVM 3.0 or greater

to

Secondly, if you are running Mac OS X with LLVM 3.5

from

In order to use the LLVM based code generator, you should install the Homebrew package manager for OS X.

to

In order to use the LLVM based code generator, you should install the Homebrew package manager for OS X and then install LLVM 3.5