Adding TOOLCHAIN_HELP to rustup help install

src/rustup-cli/help.rs

@@ -25,7 +25,19 @@ stable, beta, and nightly toolchains from the official release
 channels, then updates rustup itself.
 
 If given a toolchain argument then `update` updates that toolchain,
-the same as `rustup toolchain update`.";
+the same as `rustup toolchain update`.
+
+'toolchain' specifies a toolchain name, such as 'stable', 'nightly',
+or '1.8.0'. For more information see `rustup help toolchain`.";
+
+pub static TOOLCHAIN_INSTALL_HELP: &'static str =
+r"
+Installs a specific rust toolchain.
+
+The 'install' command is an alias for 'rustup update <toolchain>'.
+
+'toolchain' specifies a toolchain name, such as 'stable', 'nightly',
+or '1.8.0'. For more information see `rustup help toolchain`.";
 
 pub static DEFAULT_HELP: &'static str =
 r"

src/rustup-cli/rustup_mode.rs

@@ -113,6 +113,7 @@ pub fn cli() -> App<'static, 'static> {
             .after_help(SHOW_HELP))
         .subcommand(SubCommand::with_name("install")
             .about("Update Rust toolchains")
+            .after_help(TOOLCHAIN_INSTALL_HELP)
             .setting(AppSettings::Hidden) // synonym for 'toolchain install'
             .arg(Arg::with_name("toolchain")
                 .required(true)))