Help: Document if(DEFINED ENV{name})

Help/command/if.rst

@@ -187,10 +187,11 @@ Possible conditions are:
 ``if(<variable|string> IN_LIST <variable>)``
  True if the given element is contained in the named list variable.
 
-``if(DEFINED <variable>)``
- True if the given variable is defined.  It does not matter if the
- variable is true or false just if it has been set.  (Note macro
- arguments are not variables.)
+``if(DEFINED <name>|ENV{<name>})``
+ True if a variable or environment variable
+ with given ``<name>`` is defined.
+ The value of the variable does not matter.
+ Note that macro arguments are not variables.
 
 ``if((condition) AND (condition OR (condition)))``
  The conditions inside the parenthesis are evaluated first and then
@@ -270,6 +271,7 @@ A quoted or bracketed variable or keyword will be interpreted as a
 string and not dereferenced or interpreted.
 See policy :policy:`CMP0054`.
 
-There is no short form for environment or cache :ref:`Variable References`.
-They can be referenced as ``$ENV{<name>}`` or ``$CACHE{<name>}``
-wherever the above-documented condition syntax accepts <variable|string>.
+There is no automatic evaluation for environment or cache
+:ref:`Variable References`.  Their values must be referenced as
+``$ENV{<name>}`` or ``$CACHE{<name>}`` wherever the above-documented
+condition syntax accepts ``<variable|string>``.