Multi-target: NEWS and user manual

Multi-target: NEWS and user manual

This commit documents the new multi-target features in both NEWS and
user manual.

gdb/ChangeLog:
2020-01-10  Pedro Alves  <palves@redhat.com>

	* NEWS: Mention multi-target debugging, "info connections", and
	"add-inferior -no-connection".

gdb/doc/ChangeLog:
2020-01-10  Pedro Alves  <palves@redhat.com>

	* gdb.texinfo (Starting): Say "current inferior not connected"
	instead of "GDB not connected".
	(Inferiors and Programs): Rename node to ...
	(Inferiors Connections and Programs): ... this.  Update all
	references.  Talk about multiple target connections.  Update "info
	inferiors" info to mention the connections column.  Describe "info
	connections".  Document "add-inferior -no-connection".
	* guile.texi, python.texi: Update cross references.