Clearer language in BIP38

This does not change the content of the BIP, but makes things clearer to people reading the BIP in today's context (where there are multiple "bitcoin address" types that use compressed DER public keys, and taproot introduces another compressed format (x-only public keys)) These additions to the language are implied given the late-2013 early-2014 context of this BIP. This fix will make them explicit.

Specification