NEWS and documentation for alias default-args related concept and commands.

gdb/ChangeLog
2020-06-22  Philippe Waroquiers  <philippe.waroquiers@skynet.be>

	* NEWS: Mention change to the alias command.

gdb/doc/ChangeLog
2020-06-22  Philippe Waroquiers  <philippe.waroquiers@skynet.be>

	* gdb.texinfo (Command aliases default args): New node documenting
	how to use default args for a command using aliases.
	(Aliases): Document the new 'DEFAULT-ARGS...' option.
	(Help): Update help aliases text and describe when full alias
	definition is provided.