update rust.channel default value documentation

config.example.toml

@@ -578,7 +578,10 @@
 # The "channel" for the Rust build to produce. The stable/beta channels only
 # allow using stable features, whereas the nightly and dev channels allow using
 # nightly features
-#channel = "dev"
+#
+# If using tarball sources, default value for `channel` is taken from the `src/ci/channel` file;
+# otherwise, it's "dev".
+#channel = if "is a tarball source" { content of `src/ci/channel` file } else { "dev" }
 
 # A descriptive string to be appended to `rustc --version` output, which is
 # also used in places like debuginfo `DW_AT_producer`. This may be useful for

src/bootstrap/src/utils/change_tracker.rs

@@ -215,4 +215,9 @@ pub const CONFIG_CHANGE_HISTORY: &[ChangeInfo] = &[
         severity: ChangeSeverity::Info,
         summary: "Removed android-ndk r25b support in favor of android-ndk r26d.",
     },
+    ChangeInfo {
+        change_id: 125181,
+        severity: ChangeSeverity::Warning,
+        summary: "For tarball sources, default value for `rust.channel` will be taken from `src/ci/channel` file.",
+    },
 ];