Fix worker protocol document. Closes discoproject#273

Thanks to https://github.com/LukeHoersten

doc/howto/worker.rst

@@ -57,7 +57,7 @@ Message Format
 
 Messages in the protocol, both from and to the worker, are in the format:
 
-         *<name>* 'SP' *<payload-len>* 'SP' *<payload>* '\n'
+         *<name>* 'SP' *<payload-len>* 'SP' *<payload>* '\\n'
 
 where 'SP' denotes a single space character, and *<name>* is one of:
 
@@ -79,6 +79,9 @@ where 'SP' denotes a single space character, and *<name>* is one of:
 *<payload-len>* is the length of the *<payload>* in bytes,
 and *<payload>* is a :term:`JSON` formatted term.
 
+Note that messages that have no payload (see below) actually
+contain an empty JSON string *<payload> = ""* and *<payload-len> = 2*.
+
 Messages from the Worker to Disco
 =================================
 
@@ -193,12 +196,13 @@ INPUT
    each such location is called a `replica`.  A `rep_id` is an integer
    identifying the replica.
 
-   The `replica_location` is specified as a URL.  The protocol scheme
-   used for the `replica_location` could be one of `http`, `disco`, or
-   `raw`.  A URL with the `disco` scheme is to be accessed using HTTP
-   at the `disco_port` specified in the `TASK` response from Disco.
+   The `replica_location` is specified as a URL. The protocol scheme
+   used for the `replica_location` could be one of `http`, `disco`,
+   `dir` or `raw`. A URL with the `disco` scheme is to be accessed using
+   HTTP at the `disco_port` specified in the `TASK` response from Disco.
    The `raw` scheme denotes that the URL itself (minus the scheme) is
-   the data for the task.
+   the data for the task. The data needs to be properly URL encoded,
+   for instance using Base64 encoding.
 
    One important optimization is to use the local filesystem instead
    of HTTP for accessing inputs when they are local.  This can be