update rust.channel documentation

Signed-off-by: onur-ozkan <[email protected]>

Author: onur-ozkan

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