Rewrite a comment on measurement systems

CLDR does supply information on measurement system, which may be
richer than what we're currently working with, but

* I don't see any hint to which measurement system to use for each
  locale by default;
* there is some support for selecting combinations of locale and
  measurement system, suggesting it doesn't believe in such a default
  in any case; and
* even if it were there, adding it to locale_data[] would take up more
  memory than special-casing the few locales that use anything but SI.

Revise the comment to reflect this.

Change-Id: Idfa603fc9a5a55d0bd0da122ac66c76b0edf9f57
Reviewed-by: Konstantin Ritt <ritt.ks@gmail.com>
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

src/corelib/text/qlocale_data_p.h

@@ -23,9 +23,11 @@
 
 QT_BEGIN_NAMESPACE
 
-/* This part of the file isn't generated, but written by hand since
- * Unicode CLDR doesn't contain measurement system information.
- */
+/* This part of the file isn't generated, but written by hand. Unicode CLDR's
+   information about measurement systems doesn't say which to use by default in
+   each locale. Even if it did, adding another entry in every locale's row of
+   locale_data[] would take up much more memory than the small table below.
+*/
 struct TerritoryLanguage
 {
     quint16 languageId;