random() function documentation

Hi Hackers,

I just noticed that the since the random() rewrite¹, the documentation's
claim² that it "uses a simple linear congruential algorithm" is no
longer accurate (xoroshiro128** is an xorshift variant, which is a
linear-feedback shift register algorithm).

I don't have a suggestion for the exact wording, since I don't know
whether xoroshiro128** qualifies as "simple", or to what level of
specificity we want to document the algorithm.

- ilmari

[1] https://git.postgresql.org/gitweb/?p=postgresql.git;a=commit;h=3804539e48e794781c6145c7f988f5d507418fa8
[2] https://www.postgresql.org/docs/devel/functions-math.html#FUNCTIONS-MATH-RANDOM-TABLE