configure.ac: clarify description of --enable-neon

Change-Id: Id261dcf396f19a385203ae5f339cc5570efa4aab
diff --git a/configure.ac b/configure.ac
index 2397b2f..cc8d3a7 100644
--- a/configure.ac
+++ b/configure.ac
@@ -381,7 +381,7 @@
 AC_ARG_ENABLE(neon,
 	[AS_HELP_STRING(
 		[--enable-neon],
-		[Enable NEON support]
+		[Enable ARM NEON instructions support [default=no]]
 	)],
 	[neon=$enableval], [neon="no"])
 AC_DEFINE(HAVE_NEON,,