Depends. Sometimes a one-liner or a reference to a ticket is enough.

There are times when I make a one-line change and write a paragraph or two explaining why it had to be done. But these kinds of things often drown in the noise of a dozen other changes. If that one was important enough, I will reference it in an ongoing discussion or documentation, or at least include "read below:" on the first line.

I usually see people include a more elaborate commentary with the pull request. If the changeset is good but the series of individual commits is a bit messy, just merge by squashing.

(Also: this comment is meta.)