gdb/thread: fine-tune 'info threads' help text

gdb/thread: fine-tune 'info threads' help text

The help text of 'info threads' is below:

  (gdb) help info threads
  Display currently known threads.
  Usage: info threads [OPTION]... [ID]...

  Options:
    -gid
      Show global thread IDs.If ID is given, it is a space-separated list of IDs of threads to display.
  Otherwise, all threads are displayed.
  (gdb)

I think the "If ID is given ..." info should have come right below
the the usage line.  This patch reorganizes the text so that we get

  (gdb) help info threads
  Display currently known threads.
  Usage: info threads [OPTION]... [ID]...
  If ID is given, it is a space-separated list of IDs of threads to display.
  Otherwise, all threads are displayed.

  Options:
    -gid
      Show global thread IDs.
  (gdb)

gdb/ChangeLog:
2020-10-19  Tankut Baris Aktemur  <tankut.baris.aktemur@intel.com>

	* thread.c (_initialize_thread): Fine-tune the help text of
	'info threads'.