Merge #10626: doc: Remove outdated minrelaytxfee comment

Summary:
fab9b60 doc: Remove outdated minrelaytxfee comment (MarcoFalke)

Tree-SHA512: e36c5a554a9773f034c2d358c52a772688009a5a71dfd3f664d9b5566c98378f44ef7c6dc0902b42d69ab5fd54b60d4850a2903823d0b2309c7ae063d1923f7f

Backport of PR10626
https://github.com/bitcoin/bitcoin/pull/10626/

contrib/devtools/check-doc.py was used for travis, so those changes have not been included.

Test Plan:

make check

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

Reviewed By: jasonbcox, O1 Bitcoin ABC, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D3597