doc: Correct docstring describing max block tree db cache

Summary:
It is always applied in the same way, no matter how the txindex is
setup. This was no longer accurate after 8181db8, where their
initialization was made independent.

This is a partial backport of core#31483
https://github.com/bitcoin/bitcoin/pull/31483/commits/5db7d4d3d28bd1269a09955b4695135c86c4827d

Depends on D18577

Test Plan: proof-read

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D18578