fix(cli): Refer to commands, not subcommands (#16226)

### What does this PR try to resolve?

This makes us completely consistent on referring to cargo commands for
the top-level commands.

Fixes #10195

### How to test and review this PR?

Author: weihanglo

src/bin/cargo/commands/help.rs

@@ -14,7 +14,7 @@ const COMPRESSED_MAN: &[u8] = include_bytes!(concat!(env!("OUT_DIR"), "/man.tgz"
 
 pub fn cli() -> Command {
     subcommand("help")
-        .about("Displays help for a cargo subcommand")
+        .about("Displays help for a cargo command")
         .arg(Arg::new("COMMAND").action(ArgAction::Set).add(
             clap_complete::ArgValueCandidates::new(|| {
                 super::builtin()

src/bin/cargo/main.rs

@@ -212,7 +212,7 @@ fn list_commands(gctx: &GlobalContext) -> BTreeMap<String, CommandInfo> {
     commands.insert(
         "help".to_string(),
         CommandInfo::BuiltIn {
-            about: Some("Displays help for a cargo subcommand".to_string()),
+            about: Some("Displays help for a cargo command".to_string()),
         },
     );