gdb/doc: Additional information about 'info line'

gdb/doc: Additional information about 'info line'

Extend the documentation of 'info line' command to:

    1. Make 'info line' with no argument more obvious, and make it clearer
       what this does.

    2. Cover what happens when a secod 'info line' with no argument is
       issued.

    3. Extend the example output for 'info line ...' to include
       symbolic addresses.

gdb/doc/ChangeLog:

	* gdb.texinfo (Machine Code): Additional information about "info
	line" command.