Mention why 'mostly' instead of 'always'

Author: jyn514

src/building/bootstrapping.md

@@ -209,6 +209,9 @@ The `rustc` generated by the stage0 compiler is linked to the freshly-built
 so that `rustc` can use features added to std immediately after their addition,
 without need for them to get into the downloaded beta.
 
+The only time `rustc` uses `cfg(bootstrap)` is when it adds internal lints
+that use diagnostic items. This happens very rarely.
+
 ### Directories and artifacts generated by x.py
 
 The following tables indicate the outputs of various stage actions: