Many improvements, almost amounting to a partial rewrite:

Many improvements, almost amounting to a partial rewrite:
* more precision what the CIPHER_CTX functions do
* more precision what an NID is
* avoid talking about RC2, use AES-256 for an example instead
* clarify that block sizes are measured in bytes
* mention additional restrictions regarding valid block sizes
* add the missing description of the *_flags(3) functions
* mention the public mask constant EVP_CIPH_MODE
* add three missing modes that can occur as return values
* add the missing entries for *_flags(3) and *_mode(3) below RETURN VALUES
* tweak various wordings for precision and conciseness