Update throttling.md to provide context for granular throttling control through the action decorator

[144mdgross]

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Description

There were recent updates to the @action decorator calling a little more attention to the kwargs it accepts.
I thought it would be useful to also provide an example in the throttling section of how those kwargs can be used to define/override throttle_classes through the action decorator as well.

[144mdgross]

I just wanted to throw out there: this PR isn't a part of The DigitialOcean Hacktober that motivated people to submit so many one line PRs.
I had no idea that was a thing, and submitted this PR simply because I discovered this functionality by reading through the source code, and it proved very helpful. My co-workers were unaware of this functionality, so it felt important to update the docs with an example because there are probably a lot of people out there that don't realize the full potential here.
And my changes just happened to have been during October.
That's all and thanks for your work.

[xordoquy]

@144mdgross no problem, your contribution clearly isn't spam and needs some attention. I did not merge it because we recently slightly updated the action decorator documentation (though it's not pushed yet).
However, I think I'm fine since it's in the throttling page.

[xordoquy]

thanks for the contribution and sorry for the delay.

[144mdgross]

thanks for the contribution and sorry for the delay.

No worries! Thanks @xordoquy for taking the time to check it out and merge! I appreciate your time!