update "if-unchanged" comments in config.example.toml

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

Author: onur-ozkan

config.example.toml

@@ -500,8 +500,9 @@
 # This is useful if you are working on tools, doc-comments, or library (you will be able to build
 # the standard library without needing to build the compiler).
 #
-# Set this to "if-unchanged" to only download if the compiler (and library if running on CI) have
-# not been modified.
+# Set this to "if-unchanged" if you are working on `src/tools`, `tests` or `library` (on CI, `library`
+# changes triggers in-tree compiler build) to speed up the build process.
+#
 # Set this to `true` to download unconditionally.
 #download-rustc = false