Document new "hex-encoded pins" feature
diff --git a/doc/agent-api.txt b/doc/agent-api.txt
index d8d35c0..0c84e1c 100644
--- a/doc/agent-api.txt
+++ b/doc/agent-api.txt
@@ -26,7 +26,13 @@
 			needs to get the passkey for an authentication.
 
 			The return value should be a string of 1-16 characters
-			length. The string can be alphanumeric.
+			length. Longer strings are truncated to 16 characters.
+			The string can be alphanumeric.
+
+			Strings starting with '$' are parsed as hex-encoded
+			pins. That is, each two following hex characters form
+			a single byte of the resulting pin. The parser is
+			case-insensitive.
 
 			Possible errors: org.bluez.Error.Rejected
 			                 org.bluez.Error.Canceled