Athens: Add more details to the filtering docs

[arschles]

Describe the bug

We're missing a few things in the filtering docs that we've gotten some questions about (thank you @rogpeppe for bringing these up!)

• What exactly does Athens return when an exclude is found? (hint: 404)
• Does Athens include or exclude by default?
  
  
  
  • For example, how would you exclude everything by default and then only include a few things
  
  
  • And on the other hand, how would you include everything by default, and then selectively exclude
  
  

Error Message

Additional context

cc/ @rogpeppe who brought these questions up on the Athens slack today

[fedepaol]

I can grab this

[robjloranger]

@fedepaol is this one still up for grabs?

[fedepaol]

It is, sorry if I have gone missing. I really hope to get back (soon or later) but for now this is free to take

[robjloranger]

no problem, I'm only just getting back myself.

[arschles]

@fedepaol @robjloranger I know that we have a bit of filtering docs now. Do you think it's enough?

Let me know if either of you are still up for taking this on. No pressure to do so as always, just looking to see if someone else wants to do it 😄

Chill OSS projects FTW ✌️

[robjloranger]

I will do it, i need to get my toes wet again

[arschles]

Sounds great @robjloranger 👍 🎉

[robjloranger]

ok seriously i will do this now

[arschles]

😁 when you're ready, I'll be ready to review 😄

[robjloranger]

Idk when I'll get to this so leaving it up for grabs