[Docs] Add CHANGES entries for Python UnboundedSource and Watch

[Eliaaazzz]

Adds the missing CHANGES.md entries for two Python features that are already merged:

• UnboundedSource, the SDF wrapper added in [Python] Add UnboundedSource SDF wrapper (#19137) #38724, tracked by Support for custom Python UnboundedSource  #19137.
• Watch, added in [Python] Add Watch transform with growth_of polling SDF #39023, tracked by Create a Watch transform in Python #21521.

Both went in after the 2.75.0 release branch was cut, and neither sdks/python/apache_beam/io/unbounded_source.py nor sdks/python/apache_beam/io/watch.py is present on release-2.75, so the entries go under [2.76.0] - Unreleased.

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