Document the "empty defaults" behaviour of NewAPI fn

[ForestJohnson]

Description

I think webrtc.NewAPI docstring needs needs an ALL CAPS BOLD WARNING TEXT and a link to an example of how to set it up to so that the resulting webrtc.NewPeerConnection()s will behave exactly the same as the ones you get from the default webrtc.NewPeerConnection().

Reference issue

Fixes #2516

[codecov]

Codecov Report

Patch coverage has no change and project coverage change: +0.09 🎉

Comparison is base (90e3a83) 77.61% compared to head (55051d4) 77.70%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2517      +/-   ##
==========================================
+ Coverage   77.61%   77.70%   +0.09%     
==========================================
  Files          87       87              
  Lines        9357     9357              
==========================================
+ Hits         7262     7271       +9     
+ Misses       1663     1657       -6     
+ Partials      432      429       -3     

Flag	Coverage Δ
go	79.47% <ø> (+0.10%)	⬆️
wasm	70.15% <ø> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
api.go	100.00% <ø> (ø)

... and 2 files with indirect coverage changes

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[Sean-Der]

Sorry this took so long @ForestJohnson I really appreciate your feedback. I am ignorant to how it feels as a new user to use Pion. Your feedback/notes are really important. Anything else that frustrated you?

Merged with 6b22634