Document ptrauth_string_discriminator in clang/docs/PointerAuthentication.

Author: ahmedbougacha

clang/docs/PointerAuthentication.rst

@@ -328,6 +328,21 @@ be done in a single instruction with an immediate integer.
 ``pointer`` must have pointer type, and ``integer`` must have integer type. The
 result has type ``ptrauth_extra_data_t``.
 
+``ptrauth_string_discriminator``
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+.. code-block:: c
+
+  ptrauth_string_discriminator(string)
+
+Produce a discriminator value for the given string.  ``string`` must be
+a string literal of ``char`` character type.  The result has type
+``ptrauth_extra_data_t``.
+
+The result is always a constant expression.  The result value is never zero and
+always within range for both the ``__ptrauth`` qualifier and
+``ptrauth_blend_discriminator``.
+
 ``ptrauth_strip``
 ^^^^^^^^^^^^^^^^^