Every commit contains an ordered list of it's parent commits. You could think of the 'main' history as being the chain of commits that are always the first ance

Every commit contains an ordered list of it's parent commits. You could think of the 'main' history as being the chain of commits that are always the first ancestor to each other and this can be a use thing to display when generating a commit history. However, the history of a commit is always the entire tree of ancestors. This entire tree is the precise definition of what constitutes all the commits of a branch. Thus while I agree that it is not wrong to view a branch as a bookmark, I also think that attempts to nitpick the man page's use of the term fall flat here. "Commit contained in a branch" just means: "commit contained in the history of the commit the branch currently points to." and isn't ambiguous.