bootstrap.example.toml: Hint how to allow build.warnings

This commit turns "Actual" into "Expected" for the following problem:

Steps:
1. Do some quick and experimental local code changes in the compiler code.
2. Run `./x build`.
3. Observe `error: warnings are denied by `build.warnings` configuration`.
4. Decide to not care and want to allow warnings.
5. Search for `build.warnings` in **bootstrap.example.toml**

Actual:
No hits. You get frustrated because didn't learn how to allow warnings.

Expected:
You get a hit and can easily see how to allow warnings.

bootstrap.example.toml

@@ -826,7 +826,8 @@
 # in the sysroot. It is required for running nvptx tests.
 #rust.llvm-bitcode-linker = false
 
-# Whether to deny warnings in crates
+# Whether to deny warnings in crates. Set to `false` to avoid
+# error: warnings are denied by `build.warnings` configuration
 #rust.deny-warnings = true
 
 # Print backtrace on internal compiler errors during bootstrap