Clarify zero-length pattern split behavior

lib/core.mli

@@ -300,8 +300,12 @@ val split : ?pos:int -> ?len:int -> re -> string -> string list
         - : string list = ["3"; "4. Commas go brrr."]
     ]}
 
+    {6 Zero-length patterns:}
+
     Be careful when using [split] with zero-length patterns like [eol], [bow], 
-    and [eow].
+    and [eow].  Because they don't have any width, they will still be present in 
+    the result. (Note the position of the [\n] and space characters in the 
+    output.)
 
     {[
         # Re.split (Re.compile Re.eol) "a\nb";;
@@ -314,7 +318,16 @@ val split : ?pos:int -> ?len:int -> re -> string -> string list
         - : string list = ["a"; " b"]
     ]}
 
-    Note the position of the [\n] and space characters in the output.
+    Compare this to the behavior of splitting on the char itself. (Note that 
+    the delimiters are not present in the output.)
+
+    {[
+        # Re.split (Re.compile (Re.char '\n')) "a\nb";;
+        - : string list = ["a"; "b"]
+
+        # Re.split (Re.compile (Re.char ' ')) "a b";;
+        - : string list = ["a"; "b"]
+    ]}
 *)
 
 val split_gen : ?pos:int -> ?len:int -> re -> string -> string gen