Let users disable environment entries

It's helpful to be able to temporarily disable environment variables, as
opposed to having to remove (and then re-add) them entirely.

Fixes: QTCREATORBUG-20984
Change-Id: Ib0d287035b9357507c4c19faaf3a1517382506b5
Reviewed-by: hjk <hjk@qt.io>

src/libs/utils/environmentdialog.cpp

@@ -47,7 +47,8 @@ Utils::optional<EnvironmentItems> EnvironmentDialog::getEnvironmentItems(
         tr("Enter one environment variable per line.\n"
            "To set or change a variable, use VARIABLE=VALUE.\n"
            "Existing variables can be referenced in a VALUE with ${OTHER}.\n"
-           "To clear a variable, put its name on a line with nothing else on it."));
+           "To clear a variable, put its name on a line with nothing else on it.\n"
+           "To disable a variable, prefix the line with \"#\""));
 }
 
 } // namespace Utils