[m-rev.] for review: Announce recent changes to string type and string module.

[Peter Wang]

NEWS:
    Announce changes regarding ill-formed code unit sequences in
    strings.

library/string.m:
    Delete a note about ongoing work.

diff --git a/NEWS b/NEWS
index f41c9fb8e..b9f1b132a 100644
--- a/NEWS
+++ b/NEWS
@@ -462,6 +462,12 @@ Changes to the Mercury standard library:
    - foldr_substring/5
    - foldr_substring/6
 
+* The string type has been defined to permit ill-formed code unit sequences.
+  Predicates and functions in the string module now have defined behaviours
+  for strings containing ill-formed sequences. Also, some predicates and
+  functions now have defined behaviour on surrogate code points
+  (e.g.  failing or throwing an exception) that was previously undefined.
+
 * The following predicates have been added to the map module:
 
    - foldl5/12
diff --git a/library/string.m b/library/string.m
index 15800250c..a93578cce 100644
--- a/library/string.m
+++ b/library/string.m
@@ -7,7 +7,7 @@
 %---------------------------------------------------------------------------%
 %
 % File: string.m.
-% Main authors: fjh, petdr.
+% Main authors: fjh, petdr, wangp.
 % Stability: medium to high.
 %
 % This modules provides basic string handling facilities.
@@ -32,8 +32,7 @@
 % UTF-8 or UTF-16 via I/O, foreign code, and substring operations.
 % Predicates or functions that inspect strings may fail, throw an exception,
 % or else behave in some special way when an ill-formed code unit sequence is
-% encountered. Handling ill-formed sequences consistently in this module is
-% ongoing work.
+% encountered.
 %
 % Unexpected null characters embedded in the middle of strings can be a source
 % of security vulnerabilities, so the Mercury library predicates and functions
-- 
2.23.0