docs: note that type checking was introduced (#2287)

indicate that some limitation apply

docs/changes/4.0.0.rst

@@ -24,6 +24,20 @@ Changes to Supported Platforms
   later 4.x release, especially if we are faced with incompatible ecosystem
   changes in typing, Cython, etc.
 
+Typing support
+--------------
+
+Type checking support was introduced in version 4.0. While most of the library is
+now typed, further type annotations may be added throughout the 4.x release cycle.
+To improve them, we may introduce changes to the typing that do not affect
+runtime behavior, but may surface new or different errors with type checkers.
+
+.. note:: 
+
+  All type aliases in falcon are considered private, and if used should be
+  imported inside ``if TYPE_CHECKING:`` blocks to avoid possible import errors
+  after an update.
+
 .. towncrier release notes start
 
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