Add doc listing API changes to clean up for the next minor-version release.

doc/core/pending_api_cleanups.md

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+There are times when we make changes that include a temporary shim for
+backward-compatibility (e.g., a macro or some other function to preserve
+the original API) to avoid having to bump the minor version number in
+the next release.  However, when we do eventually want to release a
+feature that does change the API in a non-backward-compatible way, we
+will wind up bumping the minor version number anyway, at which point we
+can take the opportunity to clean up any pending backward-compatibility
+shims.
+
+This file lists all pending backward-compatibility changes that should
+be cleaned up the next time we are going to bump the minor version
+number:
+
+- remove `GRPC_ARG_MAX_MESSAGE_LENGTH` channel arg from
+  `include/grpc/impl/codegen/grpc_types.h` (commit `af00d8b`)