Merge #15194: Add comment describing fDisconnect behavior

Summary:
5b4283cb81b5d3023b9868d121b22b1f387a50ca Add comment describing fDisconnect behavior (Carl Dong)

Pull request description:

Motivated by @Sjors here: https://github.com/bitcoin/bitcoin/pull/14605#discussion_r248384309

Tree-SHA512: 8fc52eb4d3b5651c19c49b47fad75e8fb939cf524ada647e88d8d5aad7726052d94e500c1ebdb2a41b67bc4669ee61ff151a5cff81a52c68c900da562ef21751

Backport of core PR15194.

Test Plan:

ninja

Reviewers: O1 Bitcoin ABC, #bitcoin_abc, Fabien

Reviewed By: O1 Bitcoin ABC, #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D6474