Fix the string.mli documentation of unsafe-string (#8653)

unsafe-string is no longer the default since 4.06.

stdlib/string.mli

@@ -32,21 +32,19 @@
   substring of [s] if [len >= 0] and [start] and [start+len] are
   valid positions in [s].
 
-  OCaml strings used to be modifiable in place, for instance via the
-  {!String.set} and {!String.blit} functions described below. This
-  usage is deprecated and only possible when the compiler is put in
-  "unsafe-string" mode by giving the [-unsafe-string] command-line
-  option (which is currently the default for reasons of backward
-  compatibility). This is done by making the types [string] and
-  [bytes] (see module {!Bytes}) interchangeable so that functions
-  expecting byte sequences can also accept strings as arguments and
-  modify them.
+  Note: OCaml strings used to be modifiable in place, for instance via
+  the {!String.set} and {!String.blit} functions described below. This
+  usage is only possible when the compiler is put in "unsafe-string"
+  mode by giving the [-unsafe-string] command-line option. This
+  compatibility mode makes the types [string] and [bytes] (see module
+  {!Bytes}) interchangeable so that functions expecting byte sequences
+  can also accept strings as arguments and modify them.
 
-  All new code should avoid this feature and be compiled with the
-  [-safe-string] command-line option to enforce the separation between
-  the types [string] and [bytes].
-
- *)
+  The distinction between [bytes] and [string] was introduced in OCaml
+  4.02, and the "unsafe-string" compatibility mode was the default
+  until OCaml 4.05. Starting with 4.06, the compatibility mode is
+  opt-in; we intend to remove the option in the future.
+*)
 
 external length : string -> int = "%string_length"
 (** Return the length (number of characters) of the given string. *)