Document and extend readline-bindable functions

Document and extend readline-bindable functions

This adds readline-bindable function names to a few gdb functions that
already had key bindings.  This lets users change the bindings.

This also removes the gdb-command function.  Due to how this function
is implemented, it doesn't make sense to allow binding it.

Finally, this updates the documentation to reflect these changes.

gdb/ChangeLog
2019-11-11  Tom Tromey  <tom@tromey.com>

	* tui/tui.c (tui_initialize_readline): Add new bindable readline
	functions.

gdb/doc/ChangeLog
2019-11-11  Tom Tromey  <tom@tromey.com>

	* gdb.texinfo (TUI Keys): Document readline function names.

Change-Id: I2233779b7aefe372f19bd03c8f325733c3385e72