fix(help): Clarify install's positional

- That a version is accepted
- That you are selecting from the source a package which led to part of
  the confusion in rust-lang#4830

I wonder if we should rename our `CRATE` value names to `PKG`/`PACKAGE`

src/bin/cargo/commands/install.rs

@@ -18,7 +18,8 @@ pub fn cli() -> Command {
         .about("Install a Rust binary. Default location is $HOME/.cargo/bin")
         .arg(
             Arg::new("crate")
-                .value_name("CRATE")
+                .value_name("CRATE[@<VER>]")
+                .help("Select the package from the given source")
                 .value_parser(parse_crate)
                 .num_args(0..),
         )

tests/testsuite/cargo_install/help/stdout.log

@@ -1,9 +1,9 @@
 Install a Rust binary. Default location is $HOME/.cargo/bin
 
-Usage: cargo install [OPTIONS] [CRATE]...
+Usage: cargo install [OPTIONS] [CRATE[@<VER>]]...
 
 Arguments:
-  [CRATE]...  
+  [CRATE[@<VER>]]...  Select the package from the given source
 
 Options:
       --version <VERSION>     Specify a version to install