docs: document conflicting content length rejection

Author: Harshal96

CHANGELOG.rst

@@ -9,6 +9,8 @@ dev
 - Support for Python 3.9 has been removed.
 - Support for PyPy 3.9 has been removed.
 - `Stream.end_stream()` now raises `NoSuchStreamError` or `StreamClosedError` exceptions, instead of a generic `KeyError`.
+- Duplicate ``content-length`` headers with different values now raise ``ProtocolError``.
+  Previously, the first ``content-length`` header was accepted and later conflicting values were ignored.
 - **backfill from v4.3.0** Convert emitted events into Python `dataclass`, which introduces new constructors with required arguments.
   Instantiating these events without arguments, as previously commonly used API pattern, will no longer work.