Servers don't always get subject labels

Maybe we could change "As the server is always" to "if the server is
always"?

Also "communicate... to the client" looks like a relic of the time when
we had a dedicated label-change callback.

Let's just ditch this paragraph--it doesn't look particularly helpful to
me anyway.
Remove redundant pnfs-considerations introduction

This first sentence doesn't really tell us anything the section title
doesn't already tell us.
minor typo