patch 8.1.2251: ":term command" may not work without a shell

Problem:    ":term command" may not work without a shell.
Solution:   Add the ++shell option to :term. (closes #3340)

runtime/doc/terminal.txt

@@ -210,6 +210,10 @@ Command syntax ~
 					no window will be used.
 			++norestore	Do not include this terminal window
 					in a session file.
+			++shell		Instead of executing {command}
+					directly, use a shell, like with
+					`:!command` 		*E279*
+					{only works on Unix currently}
 			++kill={how}	When trying to close the terminal
 					window kill the job with {how}.  See
 					|term_setkill()| for the values.