rktio: update some source notes

racket/src/README

@@ -393,7 +393,8 @@ documentation for "config search paths" for more information.
 ========================================================================
 
 At a minimum, to port Racket to a new platform, edit "racket/sconfig.h"
-to provide a platform-specific compilation information.
+and "rktio/rktio_platform.h" to provide a platform-specific compilation
+information.
 
 ========================================================================
  Additional Compilation Notes

racket/src/racket/README

@@ -16,7 +16,7 @@ Some general guidelines for modying this code:
 
  * If a globally visible function is meant for exporting to embedding
    applications, extensions, or through the FFI, put the prototype in
-   "src/schemef.h" (and re-generated files with `make exports`).
+   "src/schemef.h" (and re-generate files with `make exports`).
    Functions available only to embedding applications can go in
    "scheme.h". Otherwise, put global function prototypes in
    "schpriv.h".

racket/src/rktio/README.txt

@@ -9,7 +9,9 @@ The library is meant to be
  * always non-blocking;
 
  * independent of global state, so that it works with or without
-   threads; and
+   threads;
+
+ * require no C preprocessor conditionals in client code; and
 
  * easily callable though a FFI.