Simplify reference to Base64

[dkg]

The current version explains rather too much of Base 64, while also pointing to RFC 4648. It should just leave the reference and trim out some of the redundant text.

The more complex the redundant text gets, the more the reader has to worry that this is somehow something different from standard b64.

[d3e3e3]

Seems to me it is useful to talk about implementation requirements for trailing "=" and handling of white space. But I'll trim it down a bit.