Add devdocs for important internal functions

[ToucheSir]

These may be as simple as a couple lines of comments, but as noted in #1243 (comment) they would be good to have:

• accum
• accum_param
• literal_{get|set}{property|field|index}
• _pullback and its many variations
• gradcheck
• All of https://github.com/FluxML/Zygote.jl/blob/master/src/compiler/emit.jl
• All macros in https://github.com/FluxML/Zygote.jl/blob/master/src/compiler/interface.jl
• pullback (how is this not documented already?...)
• Pullback
• All of https://github.com/FluxML/Zygote.jl/blob/master/src/compiler/reverse.jl

[DrChainsaw]

One more shower thought kinda proposal:

A PR checklist template (you know, the standard "[] Tests have been added" etc.) with a bullet saying something like "[] Every (undocumented) piece of code which took me more than five minutes to understand has a comment with the understanding I gained". Maybe a silly idea, I don't know...

[ToucheSir]

A checklist is a good idea and one we should've had for a while. Copied over Flux's in #1275 with a small admonition to document changes to existing functionality as well.