Document Map.fetch!/2 exception in the doc test

[PragTob]

👋 Thank you all for your excellent work!

This mirrors Map.fetch/2 and thought it was nice to have there for easy & quick overview purposes - so I quickly added it.

Couple of points:

• I couldn't figure out how to correctly show the map of not found in - seems difficult due to the \n\n in the error messages, grabbed the ... trick from the doc tests of Access.key!/1.
• The type system now warns that this will error in this specific test which is annoying and I'm not sure how to get rid of it:

tobi@qiqi:~/github/elixir(main)$ make test_stdlib
==> elixir (compile)
==> elixir (ex_unit)
warning: the call to map_get/2 will fail with a '{badkey,b}' exception
└─ (for doctest at) lib/map.ex:319

Running ExUnit with seed: 618006, max_cases: 48
Excluding tags: [windows: true]

I understand this isn't crucial so if the annoyance of dealing with the warning is big enough I'm happy to have this closed :)

Thanks y'all! 💚

[PragTob]

Done and thank you both! 💚

[josevalim]

💚 💙 💜 💛 ❤️