Add comment for cargo:: syntax

Signed-off-by: hi-rustin <rustin.liu@gmail.com>

src/cargo/core/compiler/custom_build.rs

@@ -691,6 +691,9 @@ impl BuildOutput {
             }
             let data = match iter.next() {
                 Some(val) => {
+                    // `cargo::` is reserved for future use or some way to indicate that there is a version mismatch
+                    // between the build script and cargo, whether its because no MSRV was specified
+                    // or someone was reading too new of documentation.
                     if val.starts_with(":") {
                         // Line started with `cargo::`.
                         bail!("invalid output in {}: `{}`\n\