Initial commit was by Linus[0], but it looks like most of the other commits[1] have been small amends by other people (apart from a couple of new chapters)
That "initial commit" was the import of the whole kernel tree into git, ignoring any previous history; that commit having been made by Linus does not mean any particular file was written by Linus.
You should instead look into historical repositories like https://archive.org/details/git-history-of-linux, which go further back; however, before Bitkeeper the authorship of each change was not tracked in detail (that historical repository IIRC does not have the detailed Bitkeeper history, but there is another repository somewhere which has it).
If I recall correctly, it was written by Linus Torvalds a long time ago. I haven't found yet when it originally appeared, but it was probably the pre-Bitkeeper days; I found a discussion about it dated from the last century.
There were many edits to it, by several people; its more recent story can be seen on the kernel git repository. Of course, if Linus disagreed with an edit, it wouldn't go in, so in a way it's still his document.
