crypto/elliptic: document the Name and names of each curve

crypto/elliptic: document the Name and names of each curve

See https://tools.ietf.org/html/rfc8422#appendix-A for a helpful table.

Also, commit to keeping them singletons, as that assumption is already
made all over the place in the ecosystem.

Fixes #34193

Change-Id: I2ec50fa18bb80e11d6101f2562df60b5e27d4f66
Reviewed-on: https://go-review.googlesource.com/c/go/+/218921
Run-TryBot: Filippo Valsorda <filippo@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>