feat(cli/help): discuss rustup toolchain install

Author: rami3l

src/cli/help.rs

@@ -118,6 +118,20 @@ pub(crate) fn toolchain_help() -> String {
     )
 }
 
+pub(crate) fn toolchain_install_help() -> String {
+    format!(
+        r"{HEADER}Discussion:{HEADER:#}
+    Some environment variables allow you to customize certain parameters
+    in toolchain installation, including:
+
+    - `RUSTUP_CONCURRENT_DOWNLOADS`: The number of concurrent downloads.
+    - `RUSTUP_DOWNLOAD_TIMEOUT`: The download timeout in seconds.
+
+    See <https://rust-lang.github.io/rustup/devel/environment-variables.html>
+    for more info."
+    )
+}
+
 pub(crate) fn toolchain_link_help() -> String {
     format!(
         r"{HEADER}Discussion:{HEADER:#}

src/cli/rustup_mode.rs

@@ -325,7 +325,7 @@ enum ToolchainSubcmd {
     },
 
     /// Install or update the given toolchains, or by default the active toolchain
-    #[command(aliases = ["update", "add"] )]
+    #[command(aliases = ["update", "add"], after_help = toolchain_install_help())]
     Install {
         #[command(flatten)]
         opts: UpdateOpts,