clarify Buf.list() and Blob.list() return value's type

As discussed in #3266

Author: stoned

doc/Type/Blob.pod6

@@ -152,7 +152,7 @@ Defined as:
 
     multi method list(Blob:D:)
 
-Returns the list of codepoints:
+Returns a C<List> of integers that can be treated as codepoints:
 
     say "zipi".encode("ascii").list; # OUTPUT: «(122 105 112 105)␤»
 

doc/Type/Buf.pod6

@@ -116,7 +116,7 @@ Defined as:
 
     multi method list(Buf:D:)
 
-Returns a C<List> of codepoints.
+Returns a C<List> of integers that can be treated as codepoints.
 
     say Buf.new(122,105,112,205).list; # OUTPUT: «(122 105 112 205)␤»