docs: update docs and add comment

doc/src/basics.md

@@ -40,8 +40,14 @@ This is the essence of `rustup`.
 
 ## Keeping rustup up to date
 
-Running `rustup update` also checks for updates to `rustup` and automatically
-installs the latest version. To manually check for updates and install the
+We control the automatic update of rustup by setting the auto-self-update configuration. 
+The auto-self-update setting supports three values: `enable` and `disable` and `check-only`.
+
+When it is `enable`, running `rustup update` will also check for updates to `rustup` and automatically install the latest version.
+If it is `disable`, the available updates is not checked and installed. 
+In addition, you can set auto-self-update to `check-only` if you want to check rustup for available updates during `rustup update` as well.
+
+To manually force check for updates and install the
 latest version of `rustup` without updating installed toolchains type `rustup
 self update`:
 
@@ -51,7 +57,7 @@ info: checking for self-updates
 info: downloading self-updates
 ```
 
-**Note**: `rustup` will automatically update itself at the end of any
+**Note**: If auto-self-update is `enable`, `rustup` will automatically update itself at the end of any
 toolchain installation as well.  You can prevent this automatic behaviour by
 passing the `--no-self-update` argument when running `rustup update` or
 `rustup toolchain install`.

src/cli/rustup_mode.rs

@@ -932,9 +932,12 @@ fn check_updates(cfg: &Cfg) -> Result<utils::ExitCode> {
 
 fn update(cfg: &mut Cfg, m: &ArgMatches<'_>) -> Result<utils::ExitCode> {
     let self_update_mode = cfg.get_self_update_mode()?;
-    let self_update = !m.is_present("no-self-update")
-        && !self_update::NEVER_SELF_UPDATE
-        && self_update_mode == "enable";
+    // Update only if rustup does **not** have the no-self-update feature,
+    // and auto-self-update is configured to **enable**
+    // and has **no** no-self-update parameter.
+    let self_update = !self_update::NEVER_SELF_UPDATE
+        && self_update_mode == "enable"
+        && !m.is_present("no-self-update");
     let forced = m.is_present("force-non-host");
     if let Some(p) = m.value_of("profile") {
         let p = Profile::from_str(p)?;