Format pass on 4.2 release notes [ci skip]

*   Leave two blank lines between sections/packages for readibility.

*   Always indent bullet points with four spaces. This allows multi-paragraph
    points to align properly (see #16103), and it is the same as the format we
    already use in CHANGELOGs. Doing this consistently sets an easy precedent
    for future contributors to follow.

*   Always put the PR/commit link on its own line in the source. If the bullet
    point spans multiple paragraphs, put it on its own paragraph at the end.

[Godfrey Chan & Juanito Fatas]