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 #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[EXE] install [OPTIONS] [CRATE]...
+Usage: cargo[EXE] install [OPTIONS] [CRATE[@<VER>]]...
 
 Arguments:
-  [CRATE]...  
+  [CRATE[@<VER>]]...  Select the package from the given source
 
 Options:
       --version <VERSION>     Specify a version to install

tests/testsuite/install.rs

@@ -1614,7 +1614,7 @@ fn inline_version_without_name() {
     cargo_process("install @0.1.1")
         .with_status(1)
         .with_stderr(
-            "error: invalid value '@0.1.1' for '[CRATE]...': missing crate name before '@'
+            "error: invalid value '@0.1.1' for '[CRATE[@<VER>]]...': missing crate name before '@'
 
 For more information, try '--help'.
 ",
@@ -1844,7 +1844,9 @@ fn install_empty_argument() {
     cargo_process("install")
         .arg("")
         .with_status(1)
-        .with_stderr_contains("[ERROR] invalid value '' for '[CRATE]...': crate name is empty")
+        .with_stderr_contains(
+            "[ERROR] invalid value '' for '[CRATE[@<VER>]]...': crate name is empty",
+        )
         .run();
 }
 
@@ -2455,7 +2457,7 @@ error: unexpected argument '--release' found
 
   tip: `--release` is the default for `cargo install`; instead `--debug` is supported
 
-Usage: cargo[EXE] install [OPTIONS] [CRATE]...
+Usage: cargo[EXE] install [OPTIONS] [CRATE[@<VER>]]...
 
 For more information, try '--help'.
 ",