Document CheckFunctionExists more clearly (#10044)

State that the function need only be available at link time.  Refer to
CheckSymbolExists for verifying that a declaration exists.

Modules/CheckFunctionExists.cmake

@@ -1,8 +1,10 @@
-# - Check if the function exists.
-# CHECK_FUNCTION_EXISTS(FUNCTION VARIABLE)
-# - macro which checks if the function exists
-#  FUNCTION - the name of the function
-#  VARIABLE - variable to store the result
+# - Check if a C function can be linked
+# CHECK_FUNCTION_EXISTS(<function> <variable>)
+#
+# Check that the <function> is provided by libraries on the system and
+# store the result in a <variable>.  This does not verify that any
+# system header file declares the function, only that it can be found
+# at link time (considure using CheckSymbolExists).
 #
 # The following variables may be set before calling this macro to
 # modify the way the check is run:
@@ -13,7 +15,7 @@
 #  CMAKE_REQUIRED_LIBRARIES = list of libraries to link
 
 #=============================================================================
-# Copyright 2002-2009 Kitware, Inc.
+# Copyright 2002-2011 Kitware, Inc.
 #
 # Distributed under the OSI-approved BSD License (the "License");
 # see accompanying file Copyright.txt for details.