Add references to the BIO_{push,pop}(3) example

Add references to the BIO_{push,pop}(3) example

The reader may not know what digest BIOs, Base64 BIOs and file BIOs are
and the relevant function names are non-obvious, hence it's not entirely
trivial to find the manuals where they are explained. With these references
a reader should be able to turn the example into actual code.

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