PR gdb/21574: Mention $SHELL and startup-with-shell on "help run"

PR gdb/21574: Mention $SHELL and startup-with-shell on "help run"

This simple patch updates the documentation of "help run" in order to
mention that the shell used to start the inferior comes from the
$SHELL environment variable.  It also mentions that this behaviour can
be disabled by using the "set startup-with-shell off" command.

gdb/ChangeLog:
2017-06-14  Sergio Durigan Junior  <sergiodj@redhat.com>

	PR gdb/21574
	* infcmd.c (_initialize_infcmd): Expand "help run" documentation
	to mention $SHELL and startup-with-shell.