BIP125: rephrase rule bitcoin#2 for clarity

bip-0125.mediawiki

@@ -51,7 +51,7 @@ transaction) that spends one or more of the same inputs if,
 
 # The original transactions signal replaceability explicitly or through inheritance as described in the above Summary section.
 
-# The replacement transaction does not contain any new unconfirmed inputs that did not previously appear in the mempool. (Unconfirmed inputs are inputs spending outputs from currently unconfirmed transactions.)
+# The replacement transaction may only include an unconfirmed input if that input was included in one of the original transactions.  (An unconfirmed input spends an output from a currently-unconfirmed transaction.)
 
 # The replacement transaction pays an absolute fee of at least the sum paid by the original transactions.