runtime: clarify Pinner doc

runtime: clarify Pinner doc

Rewrite in idiomatic form, fix a typo, break into paragraph, rephrase
slightly.

Change-Id: If146db86aa84214e61128a3bc3e9905396d44710
Reviewed-on: https://go-review.googlesource.com/c/go/+/498716
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Jonathan Amsterdam <jba@google.com>
Reviewed-by: Ansiwen <ansiwen@gmail.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Reviewed-by: Michael Knyszek <mknyszek@google.com>