address review comment

See #115558 (comment)

Author: tshepang

config.example.toml

@@ -455,6 +455,7 @@ changelog-seen = 2
 
 # Sets the number of codegen units to build the standard library with,
 # regardless of what the codegen-unit setting for the rest of the compiler is.
+# NOTE: building with anything other than 1 is known to occasionally have bugs.
 #codegen-units-std = codegen-units
 
 # Whether or not debug assertions are enabled for the compiler and standard library.