add documentation for some stream macros

CHANGES/streams

@@ -0,0 +1,8 @@
+Arc Streams (based on Racket Ports: https://docs.racket-lang.org/reference/ports.html)
+are for dealing with sources and sinks to data, whether backed by files,
+sockets or just in-memory strings.
+
+In addition to the Arc 3.1 primitives for streams documented at
+https://arclanguage.github.io/ref/io.html, Anarki provides several macros that
+help in some common situations: `fromfile`, `tofile`, `ontofile` and
+`reading`. Read more about them using the `help` command.

arc.arc

@@ -1537,6 +1537,7 @@ nothing to read. Caller is responsible for picking an unambiguous 'eof' indicato
   `(ifread-fn ,port (fn (,var) ,then) (fn () ,else)))
 
 (mac reading (var port . body)
+"Call (read) on a port, store the result in `var`, and do something with it."
   `(ifread ,var ,port (do ,@body)))
 
 (mac fromfile (f . body)