runtime: expand Pinner documentation

runtime: expand Pinner documentation

This change expands the Pinner documentation based on a few points of
feedback.
- We need a note that the Pinner has a finalizer.
- We need a note that the Pinner is safe to reuse.

I also added a note that the zero value is ready to use, and expanded
upon the use-cases of a Pinner.

Fixes #76431.

Change-Id: I312385557e67a815db05def02c1b1d7dcaa9d764
Reviewed-on: https://go-review.googlesource.com/c/go/+/726641
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Michael Knyszek <mknyszek@google.com>
Reviewed-by: Michael Pratt <mpratt@google.com>