Reword move_internal_headers changelog entry

Reword the changelog entry to tailor it for users of the library as
opposed to developers of the library.

Signed-off-by: Chris Jones <christopher.jones@arm.com>
diff --git a/ChangeLog.d/move_internal_headers.txt b/ChangeLog.d/move_internal_headers.txt
index 25e8922..8a38fe6 100644
--- a/ChangeLog.d/move_internal_headers.txt
+++ b/ChangeLog.d/move_internal_headers.txt
@@ -1,6 +1,6 @@
 API changes
    * Move internal headers.
-     All internal headers have been moved to library/ to unify them in one
-     location that is separate from the public API. This includes some files
-     that were previously not explicitly internal such as: `bn_mul.h`,
-     `aesni.h`, `padlock.h` and `entropy_poll.h`.
+     Header files that were only meant for the library's internal use and
+     were not meant to be used in application code have been moved out of
+     the include/ directory. The headers concerned are bn_mul.h, aesni.h,
+     padlock.h, entropy_poll.h and *_internal.h.