Doc: update description of random() function.

Commit 3804539 neglected to update the documentation's claim that random() uses a "linear congruential algorithm". In hopes of avoiding the same mistake if someone replaces our PRNG again, just say it's a deterministic pseudo-random number generator and leave it at that. Anybody who really wants to know can look in the source code. COMPATIBILITY NOTE FOR RELEASE NOTES: 3804539's commit message really should have mentioned that the random() sequence after setseed(), while still repeatable, will differ from what you got in pre-v15 releases. Noted by Dagfinn Ilmari Mannsåker; wording suggestion by Dean Rasheed. Discussion: https://postgr.es/m/875ynfpiru.fsf@wibble.ilmari.org

Author: tglsfdc

doc/src/sgml/func.sgml

@@ -1832,8 +1832,9 @@ repeat('Pg', 4) <returnvalue>PgPgPgPg</returnvalue>
  </table>
 
  <para>
- The <function>random()</function> function uses a simple linear
- congruential algorithm. It is fast but not suitable for cryptographic
+ The <function>random()</function> function uses a deterministic
+ pseudo-random number generator.
+ It is fast but not suitable for cryptographic
  applications; see the <xref linkend="pgcrypto"/> module for a more
  secure alternative.
  If <function>setseed()</function> is called, the series of results of