fix(help): Explain --explain

In working on rust-lang#12578, I'm focusing on each help string to decide how it
should be handled and I noticed this.  It feels weird to explain
something in terms of another command's CLI, so I took `rustc --help`s
message and added `rustc` to clarify it.

Looking back, the flag was added in rust-lang#2551 with the message we have
today.  Nothing seems to really be said about it.

In reflecting on this, I'm not 100% convinced and am open to other
opinions.

src/bin/cargo/cli.rs

@@ -564,7 +564,13 @@ See 'cargo help <command>' for more information on a specific command.\n",
         )
         .arg(flag("version", "Print version info and exit").short('V'))
         .arg(flag("list", "List installed commands"))
-        .arg(opt("explain", "Run `rustc --explain CODE`").value_name("CODE"))
+        .arg(
+            opt(
+                "explain",
+                "Provide a detailed explanation of a rustc error message",
+            )
+            .value_name("CODE"),
+        )
         .arg(
             opt(
                 "verbose",

tests/testsuite/cargo/help/stdout.log

@@ -6,7 +6,7 @@ Usage: cargo [..][OPTIONS] [COMMAND]
 Options:
   -V, --version             Print version info and exit
       --list                List installed commands
-      --explain <CODE>      Run `rustc --explain CODE`
+      --explain <CODE>      Provide a detailed explanation of a rustc error message
   -v, --verbose...          Use verbose output (-vv very verbose/build.rs output)
   -q, --quiet               Do not print cargo log messages
       --color <WHEN>        Coloring: auto, always, never