Adding deprecation notices to pgcrypto documentation

In the "Allow tests to pass in OpenSSL FIPS mode" thread [0] it was discovered
that 3DES is joining the ranks of NIST disallowed algorithms.  The attached
patch adds a small note to the pgcrypto documentation about deprecated uses of
algorithms.  I've kept it to "official" notices such as RFC's and NIST SP's.
There might be more that deserve a notice, but this seemed like a good start.

Any thoughts on whether this would be helpful?

--
Daniel Gustafsson

[0] https://postgr.es/m/2825088.1696539339@sss.pgh.pa.us