Merge #13824: doc: Remove outdated net comment

Summary:
fa365021bbb4c0865e6655100c1cbf85e2b3c7fa doc: Remove outdated net comment (MarcoFalke)

Pull request description:

`mapAddresses` and the corresponding "critsect" has been removed in 5fee401fe14aa6459428a26a82f764db70a6a0b9 more than 6 years ago. Now is probably a good time to remove this confusing comment.

Tree-SHA512: 498a403d5703da395c18a7ebb776aa6e693e59fe43a839fefd261e0a5af58621763813979d4cfbd8d1728ce73b325b82002e393cde79bdbff33e0fbf68ab6747

Backport of Core PR13824
https://github.com/bitcoin/bitcoin/pull/13824/

Test Plan:

make

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

Reviewed By: jasonbcox, O1 Bitcoin ABC, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D4774