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.