diff --git a/Help/variable/CMAKE_LANG_PLATFORM_ID.rst b/Help/variable/CMAKE_LANG_PLATFORM_ID.rst
index 1b243e3..15bb4d6 100644
--- a/Help/variable/CMAKE_LANG_PLATFORM_ID.rst
+++ b/Help/variable/CMAKE_LANG_PLATFORM_ID.rst
@@ -1,6 +1,39 @@
 CMAKE_<LANG>_PLATFORM_ID
 ------------------------
 
-An internal variable subject to change.
+Platform identification string.
 
-This is used in determining the platform and is subject to change.
+A short string unique to the platform.  Possible values
+include:
+
+::
+
+  Linux
+  Cygwin
+  MinGW
+  Darwin
+  Windows
+  FreeBSD
+  NetBSD
+  OpenBSD
+  SunOS
+  AIX
+  IRIX
+  HP-UX
+  Haiku
+  BeOS
+  QNX
+  Tru64
+  RISCos
+  SINIX
+  UNIX_SV
+  BSDOS
+  MP-RAS
+  OSF1
+  SCO_SV
+  ULTRIX
+  Xenix
+  DOS
+  OS2
+  Windows3x
+  Empty String = Unknown Platform
diff --git a/Modules/CMakePlatformId.h.in b/Modules/CMakePlatformId.h.in
index bc26c07..3c89ff2 100644
--- a/Modules/CMakePlatformId.h.in
+++ b/Modules/CMakePlatformId.h.in
@@ -1,4 +1,7 @@
-/* Identify known platforms by name.  */
+/* Identify known platforms by name. If you add a platform here, make
+ * sure to document it in Help/variable/CMAKE_LANG_PLATFORM_ID.rst as
+ * well.
+ */
 #if defined(__linux) || defined(__linux__) || defined(linux)
 # define PLATFORM_ID "Linux"