Explain in comment about listing targets in custom toolchains

Co-authored-by: rami3l <[email protected]>

src/cli/rustup_mode.rs

@@ -1104,6 +1104,7 @@ fn show(cfg: &Cfg, m: &ArgMatches) -> Result<utils::ExitCode> {
     let active_toolchain_targets: Vec<TargetTriple> = active_toolchain_name
         .and_then(|atn| match atn {
             ToolchainName::Official(desc) => DistributableToolchain::new(cfg, desc.clone()).ok(),
+            // So far, it is not possible to list targets for a custom toolchain.
             ToolchainName::Custom(_) => None,
         })
         .and_then(|distributable| {