[docs] Improve description of LLVM_BUILD_TESTS

[keith]

Summary

This makes it clear that this only has an effect if you use the all
build target.

llvm/docs/CMake.rst

**LLVM_BUILD_LLVM_DYLIB**:BOOL
   Defaults to OFF. This cannot be used in conjunction with BUILD_SHARED_LIBS.
   Tools will only be linked to the libLLVM shared library if LLVM_LINK_LLVM_DYLIB
   is also ON.
   The components in the library can be customised by setting LLVM_DYLIB_COMPONENTS
   to a list of the desired components.
   This option is not available on Windows.
 
 **LLVM_BUILD_TESTS**:BOOL
-  Build LLVM unit tests. Defaults to OFF. Targets for building each unit test
-  are generated in any case. You can build a specific unit test using the
-  targets defined under *unittests*, such as ADTTests, IRTests, SupportTests,
-  etc. (Search for ``add_llvm_unittest`` in the subdirectories of *unittests*
-  for a complete list of unit tests.) It is possible to build all unit tests
-  with the target *UnitTests*.
+  Include LLVM unit tests in the 'all' build target. Defaults to OFF. Targets
+  for building each unit test are generated in any case. You can build a
+  specific unit test using the targets defined under *unittests*, such as
+  ADTTests, IRTests, SupportTests, etc. (Search for ``add_llvm_unittest`` in
+  the subdirectories of *unittests* for a complete list of unit tests.) It is
+  possible to build all unit tests with the target *UnitTests*.
 
 **LLVM_BUILD_TOOLS**:BOOL
   Build LLVM tools. Defaults to ON. Targets for building each tool are generated
   in any case. You can build a tool separately by invoking its target. For
   example, you can build *llvm-as* with a Makefile-based system by executing *make
   llvm-as* at the root of your build directory.
 
 **LLVM_CCACHE_BUILD**:BOOL