DOCS: Improve documentation of the EVP_PKEY type

This type was previously described in a note, which is hard to find
unless you already know where to look.

This change makes the description more prominent, and allows indexing
by adding it in the NAMES section.

The EVP_PKEY description is altered to conceptually allow an EVP_PKEY
to contain a private key without a corresponding public key.  This is
related to an OTC vote:

https://mta.openssl.org/pipermail/openssl-project/2020-December/002474.html

The description of EVP_PKEY for MAC purposes is amended to fit.

Reviewed-by: Dmitry Belyavskiy <beldmit@gmail.com>
(Merged from https://github.com/openssl/openssl/pull/13629)
