doc: adjust "Replication Slot" to mention physical & logical

doc:  adjust "Replication Slot" to mention physical & logical

Much of the "Replication Slot" chapter applies to physical and logical
slots, but it was sloppy in mentioning mostly physical slots.  This
patch clarified which parts of the text apply to which slot types.

This chapter is referenced from the logical slot/subscriber chapter, so
it needs to do double duty.

Backpatch-through: master