doc update: explain that Z3 is only for clang SA - thanks to LebedevRI for the suggestion

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@371110 91177308-0d34-0410-b5e6-96231b3b80d8

docs/CMake.rst

@@ -616,8 +616,8 @@ LLVM-specific variables
   If disabled, do not try to build the OCaml and go bindings.
 
 **LLVM_ENABLE_Z3_SOLVER**:BOOL
-  If enabled, the Z3 constraint solver is activated. A recent version of the
-  z3 library needs to be available on the system.
+  If enabled, the Z3 constraint solver is activated for the Clang static analyzer.
+  A recent version of the z3 library needs to be available on the system.
 
 CMake Caches
 ============