Help: Document $ENV{VAR} syntax in cmake-language.7 manual

Help/manual/cmake-language.7.rst

@@ -312,6 +312,9 @@ inside out, e.g. ``${outer_${inner_variable}_variable}``.
 The `Variables`_ section documents the scope of variable names
 and how their values are set.
 
+An *environment variable reference* has the form ``$ENV{VAR}`` and
+is evaluated in the same contexts as a normal variable reference.
+
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