[doc] Document that :msg may be a delay

projects/main/resources/signal-docstrings/signal-options.txt

@@ -7,7 +7,7 @@ Signal options (shared by all signal creators):
 `:uid` --------- ?id of signal instance (unique to each signal  created at callsite, contrast with  `:id`)
                  Defaults to `:auto` for tracing signals, and nil otherwise
 
-`:msg` --------- Arb app-level ?message to incl. in signal: str or vec of strs to join (with `\space`)
+`:msg` --------- Arb app-level ?message to incl. in signal: str or vec of strs to join (with `\space`), may be a delay
 `:data` -------- Arb app-level ?data    to incl. in signal: usu. a map
 `:error` ------- Arb app-level ?error   to incl. in signal: platform error [2]