[mod] [DEPRECATED] :msg_ is now undocumented

Status

  While now undocumented, backwards compatibility will be retained since:
    1. The back compatibility is cheap and simple
    2. Appenders in the wild may already depend on the argument
       (incl. several community appenders)

Motivation

  The problem with :msg_ is that its behaviour can't be easily customized,
  and customization can be useful (e.g. for #356).

  So instead of hard-coding a particular :msg_ generator as before, the
  responsibility of message generation will now be handled as just another
  aspect of output generation (i.e. handled by output-fns).

  Note that output fns already have rich customization facilities, incl.
  per-appender fn and options overrides.

src/taoensso/timbre.cljc

@@ -278,7 +278,6 @@
       :?meta           ; First-arg map when it has ^:meta metadata, used as a
                          way of passing advanced per-call options to appenders
       :vargs           ; Vector of raw args provided to logging call
-      :msg_            ; Forceable - args as a string
       :timestamp_      ; Forceable - string
       :hostname_       ; Forceable - string (clj only)
       :output-fn       ; (fn [data]) -> final output for use by appenders
@@ -727,7 +726,7 @@
                data (assoc data :vargs_ (delay vargs)) ; Deprecated
                data
                (enc/assoc-nx data
-                 :msg_ (delay (default-output-msg-fn data))
+                 :msg_ (delay (default-output-msg-fn data)) ; Deprecated
                  :hash_ ; Identify unique logging "calls" for rate limiting, etc.
                  (delay
                    (hash