[docs] Document LLVM_ENABLE_IDE

[smeenai]

Summary

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

[lebedev.ri]

llvm/docs/CMake.rst

287

i don't think you want to be specific about check-llvm-* targets, maybe just check-* instead?

[smeenai]

Comment

llvm/docs/CMake.rst

 **LLVM_ENABLE_EH**:BOOL
   Build LLVM with exception-handling support. This is necessary if you wish to
   link against LLVM libraries and make use of C++ exceptions in your own code
   that need to propagate through LLVM code. Defaults to OFF.
 
 **LLVM_ENABLE_EXPENSIVE_CHECKS**:BOOL
   Enable additional time/memory expensive checking. Defaults to OFF.
 
+**LLVM_ENABLE_IDE**:BOOL
+  Tell the build system that an IDE is being used. This in turn disables the
+  creation of certain convenience build system targets, such as the various
+  ``install-*`` and ``check-llvm-*`` targets, since IDEs don't always deal well
+  with a large number of targets. This is usually autodetected, but it can be
+  configured manually to explicitly control the generation of those targets. One
+  scenario where a manual override may be desirable is when using Visual Studio
+  2017's CMake integration, which would not be detected as an IDE otherwise.
+
 **LLVM_ENABLE_PIC**:BOOL
   Add the ``-fPIC`` flag to the compiler command-line, if the compiler supports
   this flag. Some systems, like Windows, do not need this flag. Defaults to ON.
 
 **LLVM_ENABLE_RTTI**:BOOL
   Build LLVM with run-time type information. Defaults to OFF.
 
 **LLVM_ENABLE_WARNINGS**:BOOL