docs: Document RESTStream.rest_method

[edgarrmondragon]

The gap was noted in a slack conversation.

https://meltano-sdk--1824.org.readthedocs.build/en/1824/classes/singer_sdk.RESTStream.html#singer_sdk.RESTStream.rest_method

[codecov]

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Impacted Files	Coverage Δ
singer_sdk/streams/rest.py	86.84% <ø> (ø)

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