[Documentation] Document add-initializer in the Python API docs

### Describe the documentation issue

Several customers have run into the issue here: https://stackoverflow.com/questions/74139718/onnx-runtime-adding-multiple-initializers-in-python/74139719#74139719

Document this so that usage is clear from the docs

### Page / URL

https://onnxruntime.ai/docs/api/python/

Start with the Python API documentation page at https://onnxruntime.ai/docs/api/python/ and review the linked Stack Overflow example about adding multiple initializers. Done means the add-initializer usage and the relevant multiple-initializer behavior are clearly documented on that page.

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