[fidl][docs] Update request<P> to server_end:P

This CL updates the old request<P> syntax in some documentation and in a
few FIDL library doc comments.

Change-Id: If1d032f2d518f6942ab65561968aed152c9e15bc
Reviewed-on: https://fuchsia-review.googlesource.com/c/fuchsia/+/799828
Reviewed-by: Ian McKellar <ianloic@google.com>
API-Review: Ian McKellar <ianloic@google.com>
Commit-Queue: Mitchell Kember <mkember@google.com>