ENH: Mention relationship of EXECUTE_PROCESS and EXEC_PROGRAM.

Source/cmExecProgramCommand.h

@@ -80,7 +80,10 @@ public:
       "specifies a variable in which to store the output. "
       "To capture the return value of the execution, provide a RETURN_VALUE. "
       "If OUTPUT_VARIABLE is specified, then no output will go to the "
-      "stdout/stderr of the console running cmake.";
+      "stdout/stderr of the console running cmake.\n"
+      "The EXECUTE_PROCESS command is a newer more powerful version of "
+      "EXEC_PROGRAM, but the old command has been kept for compatibility."
+      ;
     }
   
   cmTypeMacro(cmExecProgramCommand, cmCommand);

Source/cmExecuteProcessCommand.h

@@ -103,7 +103,9 @@ public:
       "If more than one OUTPUT_* or ERROR_* option is given for the same "
       "pipe the precedence is not specified.  "
       "If no OUTPUT_* or ERROR_* options are given the output will be shared "
-      "with the corresponding pipes of the CMake process itself."
+      "with the corresponding pipes of the CMake process itself.\n"
+      "The EXECUTE_PROCESS command is a newer more powerful version of "
+      "EXEC_PROGRAM, but the old command has been kept for compatibility."
       ;
     }