[docs] Document LLVM_{BUILD,LINK}_LLVM_DYLIB

Summary:
Document the LLVM_BUILD_LLVM_DYLIB and LLVM_LINK_LLVM_DYLIB
CMake options, move BUILD_SHARED_LIBS out of frequently-used,
and add a note/warning to BUILD_SHARED_LIBS.

Reviewers: beanz, delcypher, mjacob

Subscribers: mjacob, llvm-commits

Differential Revision: http://reviews.llvm.org/D16208

llvm-svn: 257864