Help: Clarify/make consistent the APPLE, UNIX and WIN32 variable docs

Help/variable/APPLE.rst

@@ -1,6 +1,5 @@
 APPLE
 -----
 
-``True`` if running on OS X.
-
-Set to ``true`` on OS X.
+Set to ``True`` when the target system is an Apple platform
+(macOS, iOS, tvOS or watchOS).

Help/variable/UNIX.rst

@@ -1,7 +1,7 @@
 UNIX
 ----
 
-``True`` for UNIX and UNIX like operating systems.
-
-Set to ``true`` when the target system is UNIX or UNIX like (i.e.
-:variable:`APPLE` and :variable:`CYGWIN`).
+Set to ``True`` when the target system is UNIX or UNIX-like
+(e.g. :variable:`APPLE` and :variable:`CYGWIN`).  The
+:variable:`CMAKE_SYSTEM_NAME` variable should be queried if
+a more specific understanding of the target system is required.

Help/variable/WIN32.rst

@@ -1,6 +1,4 @@
 WIN32
 -----
 
-``True`` on Windows systems, including Win64.
-
-Set to ``true`` when the target system is Windows.
+Set to ``True`` when the target system is Windows, including Win64.