Update functions_advanced notebook (#300)

* update functions_advanced
* Various small fixes
* A few typos
* Couple of paragraphs rephrased for clarity
* fix text on sorted dict
* Add generator consumption warnings and improve examples

Expand generators section with warnings about one-time consumption,
add detailed explanations about generator return values with examples,
adjust TOC, and add test for the 'once'
decorator.

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Co-authored-by: Edoardo Baldi <edoardob90@gmail.com>

Author: despadam