Make RSA_sign.pod less confusing.

PKCS #1 v2.0 is the name of a document which specifies an algorithm
RSASSA-PKCS1-v1_5, often referred to as "PKCS #1 v1.5" after an earlier
document which specified it. This gets further confusing because the
document PKCS #1 v2.1 specifies two signature algorithms,
RSASSA-PKCS1-v1_5 and RSASSA-PSS. RSA_sign implements RSASSA-PKCS1-v1_5.

Refer to the document using the RFC number which is easier to find
anyway, and refer to the algorithm by its name.

Reviewed-by: Kurt Roeckx <kurt@roeckx.be>
Reviewed-by: Rich Salz <rsalz@openssl.org>

GH: #1474
(cherry picked from commit aa90ca11)