message: Clarify optional registry key behaviour
diff: @@ -130,7 +130,7 @@ The keys are:
  may have the markers `{crate}` and `{version}` which are replaced with the
  name and version of the crate to download. If the markers are not present,
  then the value `/{crate}/{version}/download` is appended to the end.
  -- `api`: This is the base URL for the web API. This key is optional, and if it
  +- `api`: This is the base URL for the web API. This key is optional, but if it
  is not specified, commands such as [`cargo publish`] will not work. The web
  API is described below.
  