diff --git a/m4/mpd_depends.m4 b/m4/mpd_depends.m4
index 4898f9084..a60e11b82 100644
--- a/m4/mpd_depends.m4
+++ b/m4/mpd_depends.m4
@@ -1,3 +1,8 @@
+dnl Declare a dependency of one feature on another.  If the depending
+dnl feature is disabled, the former must be disabled as well.  If the
+dnl former was explicitly enabled, abort with an error message.
+dnl
+dnl Parameters: varname1, varname2 (=dependency), errmsg
 AC_DEFUN([MPD_DEPENDS], [
 	if test x$$2 = xno; then
 		if test x$$1 = xauto; then