gdb/tui: Update help text for scroll commands

gdb/tui: Update help text for scroll commands

GDB has some commands ('+', '-', '<', and '>') for scrolling the SRC
and ASM TUI windows from the CMD window, however the help text for
these commands lists the arguments in the wrong order.

This commit updates the help text to match how GDB actually works, and
also extends the text to describe what the arguments mean, and what
the defaults are.

There should be no change in GDBs functionality after this commit.

gdb/ChangeLog:

	* tui/tui-win.c (_initialize_tui_win): Update help text for '+',
	'-', '<', and '>' commands.

Change-Id: Ib2624891de1f4ba983838822206304e4c3ed982e