Document `uppy.once` in `docs/uppy.md` (#2960)
Document `uppy.once` in `docs/uppy.md` (#2960)

Co-authored-by: Merlijn Vos <1a2e3739d7ad4f5d5398acb250fc289704a48cc9@soverin.net>
transloadit/uppy,transloadit/uppy,transloadit/uppy,transloadit/uppy