Update -Ccodegen-units docs wrt incr. comp. in rustc book.

src/doc/rustc/src/codegen-options/index.md

@@ -257,9 +257,8 @@ them in parallel. Increasing parallelism may speed up compile times, but may
 also produce slower code. Setting this to 1 may improve the performance of
 generated code, but may be slower to compile.
 
-The default, if not specified, is 16. This flag is ignored if
-[incremental](#incremental) is enabled, in which case an internal heuristic is
-used to split the crate.
+The default, if not specified, is 16 for non-incremental builds. For
+incremental builds the default is 256 which allows caching to be more granular.
 
 ## remark