Ensure class_tui is listed in the output of "help" giving the list of classes.

Ensure class_tui is listed in the output of "help" giving the list of classes.

Before this change, "help" was not showing the TUI class.
With this change:
  (gdb) help
  ...
  support -- Support facilities.
  text-user-interface -- TUI is the GDB text based interface.
  tracepoints -- Tracing of program execution without stopping the program.
  ...
  (gdb) help text-user-interface
  TUI is the GDB text based interface.
  In TUI mode, GDB can display several text windows showing
  the source file, the processor registers, the program disassembly, ...

  List of commands:

  + -- Scroll window forward.
  ...

Note that we cannot use "tui" for the fake class command name, as "tui"
is a command.

gdb/ChangeLog

2020-05-26  Philippe Waroquiers  <philippe.waroquiers@skynet.be>

	* command.h: Add comment giving the name of class_tui.
	* cli/cli-cmds.c (_initialize_cli_cmds): If TUI defined,
	create the fake command for the help for class_tui.