GHS: Add note about -T and path style

If backslashes are used for CMAKE_GENERATOR_TOOLSET, then ctest
processing will complain about COMP0010.

For example:
  Syntax error in cmake code at

    C:/Users/XXX/test_bld/Tests/CTestTestfile.cmake:253

  when parsing string

    C:\Users\XXX\bin_tools\XXX

  Invalid escape sequence \U

  Policy CMP0010 is not set: Bad variable reference syntax is an error.  Run
  "cmake --help-policy CMP0010" for policy details.  Use the cmake_policy
  command to set the policy and suppress this warning.