multiline: Clarify what counts toward max-bytes (#517)

Co-authored-by: Shivaram Lingamneni <[email protected]>

Author: progval

extensions/multiline.md

@@ -51,8 +51,10 @@ Clients MUST ignore every token with a key that they don't understand.
 
 The only defined capability key so far is:
 
-* `max-bytes` - This defines the maximum allowed total byte length of multiline batched content *REQUIRED*
-* `max-lines` - This defines the maximum allowed number of lines in a multiline batch content *RECOMMENDED*
+* `max-bytes` - This defines the maximum allowed total byte length of a multiline batch's combined message value, as defined below *REQUIRED*
+* `max-lines` - This defines the maximum allowed number of lines in a multiline batch *RECOMMENDED*
+
+Only the last parameter of PRIVMSG and NOTICE are counted toward `max-bytes`; not origin, command, or target.
 
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