Explain why we can't use -Za in MSVC headersclean check

Bring back & update comment on -Za, which was removed
in 8bf60251 (in turn reverted in commit 2dcc3059).

For the details on -Za vs -permissive, see
   https://devblogs.microsoft.com/cppblog/permissive-switch/:
and the official Microsoft documentation.

This amends 2dcc30592b.

Change-Id: Ibeb135218dd0a17eddf821ad79246906993c586f
Reviewed-by: Alexandru Croitor <alexandru.croitor@qt.io>
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

cmake/QtHeadersClean.cmake

@@ -183,6 +183,9 @@ function(qt_internal_add_headersclean_target module_target module_headers)
         )
         set(input_header_path_type ABSOLUTE)
     elseif ("${CMAKE_CXX_COMPILER_ID}" STREQUAL "MSVC")
+        # Note we can't enable -Za, as it does not support certain key Microsoft SDK header files
+        # we use. Microsoft suggests to use /permissive- instead, which is implicity set by
+        # -std:c++latest.
         set(hcleanFLAGS -std:c++latest -Zc:__cplusplus -WX -W3 -EHsc)
 
         # Because we now add `-DNOMINMAX` to `PlatformCommonInternal`.