PR gdb/21122: Fix documentation mistakes for breakpoint commands

Currently, the breakpoint documentation refers to some commands taking breakpoint
"ranges" as arguments. We discussed this with Pedro and concluded that it would
be more accurate to speak in terms of breakpoint "lists", whose elements can optionally
be ranges. I also fixed a couple of minor mistakes in the docs.

gdb/ChangeLog:

	* breakpoint.c (_initialize_breakpoint): Update the help description
	of the 'commands' command to indicate that it takes a list argument.

gdb/doc/ChangeLog:

	* gdb.texinfo (Breakpoints): Reword documentation to speak in terms of
	space-separated breakpoint lists.  Also add a missing @table command
	and @cindex for breakpoint lists.

gdb/testsuite/ChangeLog:

	* gdb.base/help.exp: Update match pattern for testing 'help commands'.