Git was made with the intention that commit messages look like whole emails, describing not only why the change was made but also the thought process behind it, why this particular solution was chosen instead of some other etc.
What you describe is a decent header, although most people would probably prefer "Add ability to" (not "Added ability to", this is a custom that goes way back before git).
Inline documentation, or comments, is something else entirely. That is a moving target that can describe intended usage, remote APIs, and hard to understand passages. Commit messages describes a particular changeset, at a fixed point in time.
Google it, and see that most people's ideas of a good commit message goes way beyond the header ("shortlog") line.