Clarify cfg(bootstrap) docs

Author: jyn514

src/building/bootstrapping.md

@@ -256,6 +256,10 @@ So the stage2 compiler has to recompile `std` for the target.
 
 ### Why does only libstd use `cfg(bootstrap)`?
 
+NOTE: for docs on `cfg(bootstrap)` itself, see [Complications of Bootstrapping][complications].
+
+[complications]: #complications-of-bootstrapping
+
 The `rustc` generated by the stage0 compiler is linked to the freshly-built
 `std`, which means that for the most part only `std` needs to be cfg-gated,
 so that `rustc` can use features added to std immediately after their addition,
@@ -265,7 +269,7 @@ Note this is different from any other Rust program: stage1 `rustc`
 is built by the _beta_ compiler, but using the _master_ version of libstd!
 
 The only time `rustc` uses `cfg(bootstrap)` is when it adds internal lints
-that use diagnostic items. This happens very rarely.
+that use diagnostic items, or when it uses unstable library features that were recently changed.
 
 ### What is a 'sysroot'?