Merge #10380: [doc] Removing comments about dirty entries on txmempool

Summary:
4f3782e [doc] Removing comments about dirty entries on txmempool (Simone Madeo)

Tree-SHA512: 12ffad7237e2a8ec658cfcff263dad5a2220beadda48d67095df90c2f8ea55ffb6891d919698893b625965fb3069a05e7901fac6cceb3d1228e6833356408074

Backport of Core PR10380
https://github.com/bitcoin/bitcoin/pull/10380/

Test Plan:

make check

Reviewers: deadalnix, Fabien, jasonbcox, markblundeberg, O1 Bitcoin ABC, #bitcoin_abc

Reviewed By: deadalnix, Fabien, jasonbcox, O1 Bitcoin ABC, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D3385