async_to_sync should find and replace sections anywhere in the docstring

[weaverba137]

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, 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.