crypto/ecdsa,crypto/elliptic: update docs and spec references

crypto/ecdsa,crypto/elliptic: update docs and spec references

crypto/ecdsa was long overdue a cleanup. Bump the FIPS 186 version, and
make sure we consistently reference that and SEC 1, not the paywalled
ANSI standard.

Change-Id: Idd90bd6c14b334941fdcd829d89b796a60a8b174
Reviewed-on: https://go-review.googlesource.com/c/go/+/352529
Run-TryBot: Filippo Valsorda <filippo@golang.org>
Trust: Filippo Valsorda <filippo@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Roland Shoemaker <roland@golang.org>