Help: Explicitly state that if(ENV{some_var}) is always false

Help/command/if.rst

@@ -61,7 +61,9 @@ Basic Expressions
 
 ``if(<variable|string>)``
  True if given a variable that is defined to a value that is not a false
- constant.  False otherwise.  (Note macro arguments are not variables.)
+ constant.  False otherwise.  Note that macro arguments are not variables.
+ Environment variables also cannot be tested this way, e.g.
+ ``if(ENV{some_var})`` will always evaluate to false.
 
 Logic Operators
 """""""""""""""