[docs] Document object instantiation

[HT154]

This content is "reverse engineered" entirely from using the language and is not an interpretation of the code, so it may be missing or misconstruing things. Please let me know if anything needs adjusting!

[HT154]

I've almost entirely rewritten this to incorporate the great feedback @stackoverflow @holzensp

Notable callouts:

• I've added a separate section in the introduction on default values
• I broke the three cases out into their own sections and reorganized them
• I've added code blocks with examples for each instantiation type—I was a little conservative in the name of brevity, so if you see a case I didn't cover but think should be included, please let me know