Better error position explanation

Based on @Mingun's help.

README.md

@@ -458,9 +458,12 @@ character class that failed to match:
 > Expected [0-9] but "a" found.
 
 Aside from the text content of messages, human-readable names also have a
-subtler effect on *where* errors are reported. PEG.js prefers shorter matches
+subtler effect on *where* errors are reported. PEG.js prefers to match
-for named rules and longer matches for unnamed rules. For example, for this
+named rules completely or not at all, but not partially. Unnamed rules,
-rule matching a comma-separated list of integers:
+on the other hand, can produce an error in the middle of their
+subexpressions.
+
+For example, for this rule matching a comma-separated list of integers:
 
     seq
       = integer ("," integer)*