message: Update documentation on config via environment variables
  Use source replacement as a specific example of the kind
  of configuration feature that cannot currently be set by
  environmental variables.
diff: @@ -133,9 +133,11 @@ example the `build.jobs` key can also be defined by `CARGO_BUILD_JOBS`.
  Environment variables will take precedent over TOML configuration, and currently
  only integer, boolean, and string keys are supported to be defined by
  -environment variables.
  +environment variables. This means that [source replacement][source], which is expressed by
  +tables, cannot be configured through environment variables.
  In addition to the system above, Cargo recognizes a few other specific
  [environment variables][env].
  [env]: reference/environment-variables.html
  +[source]: (reference/source-replacement.html)
  