gdb/doc: Clarify vCont packet description

gdb/doc: Clarify vCont packet description

Specifically, what happens with multiple actions that could match a
thread, and what happens when we get a vCont action that matches a
thread that was already running.  E.g., what does:

 "vCont;s:2"
 "vCont;s:1;c"

mean for thread 2.

(Thread 2 continues stepping.)

gdb/doc/ChangeLog:
2016-10-26  Pedro Alves  <palves@redhat.com>

	* gdb.texinfo (Packets): Clarify vCont packets with multiple
	actions that match a thread, and what happens when an action
	matches a thread that is already running.