[docs] Document LLVM_ENABLE_IDE

Use some of the wording and the motivating example from r344555. The
lack of documentation was pointed out by Roman Lebedev.

Differential Revision: https://reviews.llvm.org/D58286

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@354167 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed