BUG: Fix <CONFIG>_POSTFIX property/variable docs

The CMAKE_<CONFIG>_POSTFIX variable and <CONFIG>_POSTFIX property were
not documented.  This updates the CMAKE_DEBUG_POSTFIX and DEBUG_POSTFIX
documentation to refer to the more general variable/property.  It also
clarifies that the variable is used as the property default only for
non-executable targets.  See issue #7868.

Source/cmDocumentVariables.cxx

@@ -822,18 +822,23 @@ void cmDocumentVariables::DefineVariables(cmake* cm)
      "See that target property for additional information.",
      false,
      "Variables that Control the Build");
-  
 
   cm->DefineProperty
     ("CMAKE_DEBUG_POSTFIX", cmProperty::VARIABLE,
-     "A postfix to add to targets when build as debug.",
-     "This variable is used to initialize the DEBUG_POSTFIX "
-     "property on all the targets. If set the postfix will be "
-     "appended to any targets built when the configuration is "
-     "Debug.",
+     "See variable CMAKE_<CONFIG>_POSTFIX.",
+     "This variable is a special case of the more-general "
+     "CMAKE_<CONFIG>_POSTFIX variable for the DEBUG configuration.",
      false,
      "Variables that Control the Build");
-  
+  cm->DefineProperty
+    ("CMAKE_<CONFIG>_POSTFIX", cmProperty::VARIABLE,
+     "Default filename postfix for libraries under configuration <CONFIG>.",
+     "When a non-executable target is created its <CONFIG>_POSTFIX "
+     "target property is initialized with the value of this variable "
+     "if it is set.",
+     false,
+     "Variables that Control the Build");
+
   cm->DefineProperty
     ("CMAKE_BUILD_WITH_INSTALL_RPATH", cmProperty::VARIABLE,
      "Use the install path for the RPATH",

Source/cmTarget.cxx

@@ -135,10 +135,19 @@ void cmTarget::DefineProperties(cmake *cm)
 
   cm->DefineProperty
     ("DEBUG_POSTFIX", cmProperty::TARGET,
-     "A postfix that will be applied to this target when build debug.",
-     "A property on a target that specifies a postfix to add to the "
-     "target name when built in debug mode. For example \"foo.dll\" "
-     "versus \"fooD.dll\".  Ignored for Mac Frameworks and App Bundles.");
+     "See target property <CONFIG>_POSTFIX.",
+     "This property is a special case of the more-general <CONFIG>_POSTFIX "
+     "property for the DEBUG configuration.");
+
+  cm->DefineProperty
+    ("<CONFIG>_POSTFIX", cmProperty::TARGET,
+     "Postfix to append to the target file name for configuration <CONFIG>.",
+     "When building with configuration <CONFIG> the value of this property "
+     "is appended to the target file name built on disk.  "
+     "For non-executable targets, this property is initialized by the value "
+     "of the variable CMAKE_<CONFIG>_POSTFIX if it is set when a target is "
+     "created.  "
+     "This property is ignored on the Mac for Frameworks and App Bundles.");
 
   cm->DefineProperty
     ("EchoString", cmProperty::TARGET,