random: fixes read buffer resizing in RandAddSeedPerfmon

Summary:
+ Replaces std::max with std::min to resize buffer in RandAddSeedPerfmon
+ Documents behavior of RandAddSeedPerfmon
This is a backport of core#20082

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D10539