update build.vendor documentation

Signed-off-by: onur-ozkan <work@onurozkan.dev>

Author: onur-ozkan

config.example.toml

@@ -311,9 +311,8 @@
 # Indicate whether the vendored sources are used for Rust dependencies or not.
 #
 # Vendoring requires additional setup. We recommend using the pre-generated source tarballs if you
-# want to use vendoring. See
-# https://forge.rust-lang.org/infra/other-installation-methods.html#source-code.
-#vendor = false
+# want to use vendoring. See https://forge.rust-lang.org/infra/other-installation-methods.html#source-code.
+#vendor = if "is a tarball source" && "vendor" dir exists && ".cargo/config.toml" file exists { true } else { false }
 
 # Typically the build system will build the Rust compiler twice. The second
 # compiler, however, will simply use its own libraries to link against. If you