[DOC] Mention what happens when the thread of a thread-specific breakpoint disappears.

[DOC] Mention what happens when the thread of a thread-specific breakpoint disappears.

We recently made GDB auto-delete thread-specific breakpoints when the
corresponding thread is removed from the thread list, but we hadn't
mentioned it in the manual.

gdb/
2013-10-07  Pedro Alves  <palves@redhat.com>

	PR breakpoints/11568
	* gdb.texinfo (Thread-Specific Breakpoints): Mention what happens
	when the thread is removed from the thread list.