ENH: Clarified documentation of EXISTS and IS_DIRECTORY modes.

Source/cmIfCommand.h

@@ -126,9 +126,11 @@ public:
       "True if the given name is a command that can be invoked.\n"
       "  IF(EXISTS file-name)\n"
       "  IF(EXISTS directory-name)\n"
-      "True if the named file or directory exists.\n"
+      "True if the named file or directory exists.  "
+      "Behavior is well-defined only for full paths.\n"
       "  IF(IS_DIRECTORY directory-name)\n"
-      "True if the given name is a directory.\n"
+      "True if the given name is a directory.  "
+      "Behavior is well-defined only for full paths.\n"
       "  IF(variable MATCHES regex)\n"
       "  IF(string MATCHES regex)\n"
       "True if the given string or variable's value matches the given "