rl-next.md: Minor improvement

(cherry picked from commit 37c533ed27)

doc/manual/src/release-notes/rl-next.md

@@ -3,8 +3,8 @@
 * A new function `builtins.readFileType` is available. It is similar to
   `builtins.readDir` but acts on a single file or directory.
 
-* The `builtins.readDir` function has been optimized when encountering unknown
+* The `builtins.readDir` function has been optimized when encountering not-yet-known
   file types from POSIX's `readdir`. In such cases the type of each file is/was
   discovered by making multiple syscalls. This change makes these operations
   lazy such that these lookups will only be performed if the attribute is used.
-  This optimization effects a minority of filesystems and operating systems.
+  This optimization affects a minority of filesystems and operating systems.