async_to_sync should find and replace sections anywhere in the docstring

While working on #2663, I discovered that `async_to_sync` and related decorators appear to assume that the `Returns` section is the last section in the docstring. However, if there are more sections than just `Parameters` and `Returns`, and one is following the [numpydoc standard](https://numpydoc.readthedocs.io/en/latest/format.html), then there is a good chance that `Returns` is *not* the last section.

#2598 suggests eliminating separate sync and async functions entirely, and that would certainly solve this issue too.

However if #2598 is not adopted, or if adoption takes a long time, then `async_to_sync` should be made more robust and be able to find the `Returns` section anywhere in the docstring.

Locate the implementation of async_to_sync and its related decorators, then inspect how they identify the Returns section in a numpydoc-style docstring. Verify the behavior with a docstring containing sections after Returns; done means Returns is found and replaced wherever it appears without disrupting later sections.

Written by the indexing model from the issue text.