Document "main" key support on Action

In theory, we support aliasing a function with a name other than "main" to "main" as this is what our runtime containers expect.

Verify this is supported (curosry look at code says we do), but should have unit tests for at least 2 languages (e.g., JS and Python and perhaps Java which was added recently).

In addition, we should have at least 1 integration test to prove it works.

Start by tracing how Action aliases a function name to "main" and inspect the existing JavaScript, Python, and Java unit-test coverage. Add unit tests for at least two languages and one integration test demonstrating the aliasing behavior, then confirm the tests pass.

Written by the indexing model from the issue text.