Document changes to input transformers

docs/source/whatsnew/pr/incompat-inputsplitter-source-raw-reset.rst

@@ -0,0 +1,6 b''
+* :class:`IPython.core.inputsplitter.IPythonInputSplitter` no longer has a method
+  ``source_raw_reset()``, but gains :meth:`~IPython.core.inputsplitter.IPythonInputSplitter.raw_reset`
+  instead. Use of ``source_raw_reset`` can be replaced with::
+  
+      raw = isp.source_raw
+      transformed = isp.source_reset()

docs/source/whatsnew/pr/inputtransformer-syntaxerrors.rst

@@ -0,0 +1,4 b''
+* Input transformers (see :doc:`/config/inputtransforms`) may now raise
+  :exc:`SyntaxError` if they determine that input is invalid. The input
+  transformation machinery in IPython will handle displaying the exception to
+  the user and resetting state.

docs/source/config/inputtransforms.rst

@@ -43,6 +43,14 b' to tell when a block of input is complete, and'
 to transform complete cells. If you add a transformer, you should make sure that
 it gets added to both.
 
+These transformers may raise :exc:`SyntaxError` if the input code is invalid, but
+in most cases it is clearer to pass unrecognised code through unmodified and let
+Python's own parser decide whether it is valid.
+
+.. versionchanged:: 2.0
+
+   Added the option to raise :exc:`SyntaxError`.
+
 Stateless transformations
 -------------------------