message: Clarify docs of install without <crate>
diff: @@ -89,9 +89,8 @@ enables overwriting existing binaries. Thus you can reinstall a crate with
  `cargo install --force <crate>`.
  Omitting the <crate> specification entirely will install the crate in the
  -current directory. That is, `install` is equivalent to the more explicit
  -`install --path .`. This behaviour is deprecated, and no longer supported as
  -of the Rust 2018 edition.
  +current directory. This behaviour is deprecated, and it no longer works in the
  +Rust 2018 edition. Use the more explicit `install --path .` instead.
  If the source is crates.io or `--git` then by default the crate will be built
  in a temporary target directory. To avoid this, the target directory can be
  