Updates To Compressed Keys Based On Reviewer Feedback

Several reviewers kindly provided feedback via IRC tonight, resulting in
the following changes:

* Mention that compressed keys are used in the offical secp256k1
  documentation with the prefix bytes used by Bitcoin. I could not find
  a publicly-available officialish document defining the prefix bytes,
  but they're used repeatedly in the offical SECG documentation we
  autoxref "secp256k1" to, so I went with that.

* Remove the mention that Bitcoin Core uses OpenSSL. While true, there
  is an effort to slowly move away from using OpenSSL for EC in
  Bitcoin Core.

* Change the phrasing of the relative curve location for 0x02 & 0x03 to
  a form more correct for actuality instead of the illustrated
  abstraction.

* Drop quotes around "uncompressed" since that's the term used in the
  official secp256k1 docs, not something Bitcoin-specific.