message: doc/manifest: clarify the meaning of the URLs
diff: @@ -109,7 +109,9 @@ There are a number of optional metadata fields also accepted under the
  # uploaded to crates.io (aka this is not markdown).
  description = "..."
  -# These URLs point to more information about the repository.
  +# These URLs point to more information about the repository. These are
  +# intended to be webviews of the relevant data, not necessarily compatible
  +# with VCS tools and the like.
  documentation = "..."
  homepage = "..."
  repository = "..."
  