Help: clarify availability of the MSVC_IDE variable

Help/variable/MSVC_IDE.rst

@@ -5,3 +5,10 @@ MSVC_IDE
 
 Set to ``true`` when the target platform is the Microsoft Visual C++ IDE, as
 opposed to the command line compiler.
+
+.. note::
+
+  This variable is only available after compiler detection has been performed,
+  so it is not available to toolchain files or before the first
+  :command:`project` or :command:`enable_language` call which uses an
+  MSVC-like compiler.