[backport#13531] doc: Clarify that mempool txiter is const_iterator

Summary:
https://github.com/bitcoin/bitcoin/pull/13531/commits/faca0a862558cf17777f36c38452ba770227cbc9

Backport Core PR13531

Test Plan:

ninja

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D6538