doc: fill out net/http.Request.PathValue,SetPathValue section

doc: fill out net/http.Request.PathValue,SetPathValue section

And add a missing code tag wrap elsewhere.

Updates #61410
Updates #61422

Change-Id: I70a9c4ecaf4056af2e88d777b8db892a45dfcb9f
Reviewed-on: https://go-review.googlesource.com/c/go/+/552195
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Run-TryBot: Dan Peterson <danp@danp.net>
Reviewed-by: Jonathan Amsterdam <jba@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>