--- a/src/code/lispinit.lisp
+++ b/src/code/lispinit.lisp
@@ -29,7 +29,7 @@
 (export '(compiler-version scrub-control-stack *runtime-features*))
 
 (defvar *runtime-features* nil
-  "Features affecting the runtime")
+  "Features affecting the runtime.  These are written to internals.h.")
 
 (in-package :extensions)
 (export '(quit *prompt*))

--- a/src/code/sysmacs.lisp
+++ b/src/code/sysmacs.lisp
@@ -20,12 +20,14 @@
 	  register-lisp-feature register-lisp-runtime-feature))
 
 (defmacro register-lisp-feature (feature)
-  "Register the feature as having influenced the CMUCL build process."
+  "Register the feature as having influenced the CMUCL build
+  process. Feature is added to *feature*"
   `(pushnew ,feature *features*))
 
 (defmacro register-lisp-runtime-feature (feature)
   "Register the feature as having influenced the CMUCL build process,
-and also the CMUCL C runtime."
+and also the CMUCL C runtime. Feature is added to*features* and
+sys::*runtime-features*."
   (let ((f (gensym)))
     `(progn
       (let ((,f ,feature))