Help: UNKNOWN_LIBRARY is a valid file API target type

This was missed in the change that added imported targets to the
file API replies, b626843d71 (fileAPI: Output all INTERFACE and
IMPORTED targets, 2025-09-13). The release notes and file API schema
did get the update to mention it, only the description of the `type`
field in the file API docs was missed.

Help/manual/cmake-file-api.7.rst

@@ -1113,6 +1113,12 @@ with members:
   ``MODULE_LIBRARY``, ``OBJECT_LIBRARY``, ``INTERFACE_LIBRARY``,
   or ``UTILITY``.
 
+  .. codemodel-versionchanged:: 2.9
+
+    ``UNKNOWN_LIBRARY`` is now also a possible value.  It corresponds to a
+    library created using :command:`add_library(IMPORTED)` with the ``UNKNOWN``
+    library type.
+
 ``imported``
   .. codemodel-versionadded:: 2.9