document when version detection will not work

Modules/FindGnuplot.cmake

@@ -5,6 +5,8 @@
 #  GNUPLOT_FOUND - system has Gnuplot
 #  GNUPLOT_EXECUTABLE - the Gnuplot executable
 #  GNUPLOT_VERSION_STRING - the version of Gnuplot found (since CMake 2.8.8)
+#
+# GNUPLOT_VERSION_STRING will not work for old versions like 3.7.1.
 
 #=============================================================================
 # Copyright 2002-2009 Kitware, Inc.

Modules/FindImageMagick.cmake

@@ -27,6 +27,8 @@
 #  ImageMagick_VERSION_STRING         - the version of ImageMagick found
 #                                       (since CMake 2.8.8)
 #
+# ImageMagick_VERSION_STRING will not work for old versions like 5.2.3.
+#
 # There are also components for the following ImageMagick APIs:
 #
 #  Magick++