[gdb/doc] Improve info line command documentationHEADmaster

The "info line" command can be used with or without arguments.

About using it without arguments, the documentation [1] says the following:
...
info line

    With no locspec, information about the current source line is printed.

  ...

After info line, using info line again without specifying a location will
display information about the next source line.
...

That does not describe the fact that the command may also show the last line
listed:
...
(gdb) info line
No line number information available.
(gdb) list main
20	#include "interps.h"
21	#include "run-on-main-thread.h"
22
23	int
24	main (int argc, char **argv)
25	{
26	  /* The first call to is_main_thread () should be from the main thread.
27	     If this is the first call, then that requirement is fulfilled here.
28	     If this is not the first call, then this verifies that the first call
29	     fulfilled that requirement.  */
(gdb) info line
Line 29 of "gdb/gdb.c" is at address 0x419684 <main(int, char**)+30>
   but contains no code.
(gdb)
...

Update the documentation to list this additional functionality.

Approved-By: Eli Zaretskii <eliz@gnu.org>

Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=33783

[1] https://www.sourceware.org/gdb/current/onlinedocs/gdb.html/Machine-Code.html#index-info-line

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