Add description to the precious Python variable.

configure.ac

@@ -241,7 +241,7 @@ AM_CONDITIONAL(MINIUPNPC, test x$upnp = xtrue)
 AM_PROG_CC_C_O
 AC_PROG_CC_C99
 
-AC_ARG_VAR(PYTHON)
+AC_ARG_VAR([PYTHON], [path to Python binary])
 AC_CHECK_PROGS(PYTHON, [python python2 python2.7 python3 python3.3])
 if test "x$PYTHON" = "x"; then
   AC_MSG_WARN([Python unavailable; some tests will not be run.])