wireprototypes: clarify documentation of getbundle argument types

It seems like it was a mix of what the Python code would see and what
was sent over the wire. I've tried to clarify both the type seen in
Python and how it's transmitted.

Differential Revision: https://phab.mercurial-scm.org/D6871

mercurial/wireprototypes.py

@@ -147,14 +147,14 b' def unescapebatcharg(escaped):'
 
 # mapping of options accepted by getbundle and their types
 #
-# Meant to be extended by extensions. It is extensions responsibility to ensure
-# such options are properly processed in exchange.getbundle.
+# Meant to be extended by extensions. It is the extension's responsibility to
+# ensure such options are properly processed in exchange.getbundle.
 #
 # supported types are:
 #
-# :nodes: list of binary nodes
-# :csv:   list of comma-separated values
-# :scsv:  list of comma-separated values return as set
+# :nodes: list of binary nodes, transmitted as space-separated hex nodes
+# :csv:   list of values, transmitted as comma-separated values
+# :scsv:  set of values, transmitted as comma-separated values
 # :plain: string with no transformation needed.
 GETBUNDLE_ARGUMENTS = {
     'heads':  'nodes',