docs: explain the limitation of IPython magic dict arguments

[plamut]

Closes#108.

This PR clarifies the limitation of passing a dictionary argument to IPython cell magic, and documents a workaround.

Fixing the root cause is outside of BigQuery's scope, as it would require modifications to IPython's own argument parsing logic.

PR checklist

• Make sure to open an issue as a bug/issue before writing your code! That way we can discuss the change, evaluate designs, and agree on the general idea
• Ensure the tests and linter pass
• Code coverage does not decrease (if any source code was changed)
• Appropriate docs were updated (if necessary)