Merge #15343: [doc] netaddress: Make IPv4 loopback comment more descriptive

Summary:
87aa0b48afe5a070e6e11e502a1803da6add5197 netaddress: Make IPv4 loopback comment more descriptive (Carl Dong)
6180b5f32bbcd7ef18db772290954161806e609e netaddress: Fix indentation in IsLocal (Carl Dong)

Pull request description:

This also makes the comment match the IPv6 comment just below this hunk.

Tree-SHA512: 9b91195e71e18156c9e013f63a6d430c67951aabb4a0c2f48f3bf852570c13887572b9e2fa52f4e1beba8685a9cae8949d4d03cd618a78f88566cf9e85dc64a8

Backport of Core PR15343

Test Plan:

ninja

Reviewers: O1 Bitcoin ABC, #bitcoin_abc, Fabien

Reviewed By: O1 Bitcoin ABC, #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D6476