add some documentation regarding MPI protocol

remove link to ESC sequence documentation (since we no longer use literal ESC
for delimiters) and replace with link to control codes on wikipedia (slightly
redundant with existing link but with more detail).

Update docstring for ess--mpi-handle-messages to
1. Use octal's for literals for better readability on github
2. add a note calling attention to the fact that the message contains literal
ASCII control codes which hopefully gives something search-able if a user is
confused by the seemingly strange characters printed in the docstring

Author: dankessler

lisp/ess-tracebug.el

@@ -1182,10 +1182,10 @@ Kill the *ess.dbg.[R_name]* buffer."
 ;;; MPI
 
 ;; http://jkorpela.fi/chars/c0.html
-;; https://en.wikipedia.org/wiki/ANSI_escape_code#Escape_sequences
-(defvar ess--mpi-message-start-delimiter "\001")
-(defvar ess--mpi-message-field-separator "\002")
-(defvar ess--mpi-message-end-delimiter "\003")
+;; https://en.wikipedia.org/wiki/C0_and_C1_control_codes
+(defvar ess--mpi-message-start-delimiter "\001") ; SOH control code
+(defvar ess--mpi-message-field-separator "\002") ; STX control code
+(defvar ess--mpi-message-end-delimiter "\003")   ; ETX control code
 
 (define-obsolete-variable-alias 'ess-mpi-alist 'ess-mpi-handlers "ESS 19.04")
 (defvar ess-mpi-handlers
@@ -1229,10 +1229,12 @@ value from EXPR and then sent to the subprocess."
 
 (defun ess--mpi-handle-messages (buf)
   "Handle all mpi messages in BUF and delete them.
-The MPI message has the form 
TYPEFIELD... where TYPE is the
-type of the messages on which handlers in `ess-mpi-handlers' are
-dispatched. And FIELDs are strings. Return :incomplete if BUF
-ends with an incomplete message."
+The MPI message has the form \001TYPE\002FIELD...\003 where TYPE
+is the type of the messages on which handlers in
+`ess-mpi-handlers' are dispatched. And FIELDs are strings. Note
+that the MPI message contains literal ASCII control codes as
+delimiters. Return :incomplete if BUF ends with an incomplete
+message."
   (let ((obuf (current-buffer))
         (out nil))
     (with-current-buffer buf