Explicit documentation for deprecated PKCS#1v15 Encryption

[timimm]

Usage of the deprecated PKCS#1 v1.5 encryption algorithm (i.e., RSAES encryption scheme) should be explicitly documented.

[tarcieri]

It would be good to talk about Bleichenbacher attacks in the documentation

[tarcieri]

I guess you were just asking for a usage example? There's one here:

https://docs.rs/rsa/0.10.0-rc.13/rsa/#pkcs1-v15-encryption

Although if you were asking why the docs don't mention PKCS#1 v1.5 is deprecated, here's a PR to address that: #637