…I PR

…ut IO enabled.

I was initially trying to get Terraform to ignore settings that wern't
set in the configuration, leaving them at their previously configured
value. This seems to be counter to Terraform's design, though, so I've
relented & included the current default values for each setting here.

It seems like maybe we could have done that for the string values, and
non-0 integers, but it seems like a nonstarter for booleans
(hashicorp/terraform-plugin-sdk#817), and
I didn't want to go against Terraform's design too much.

Part of this is updating field with descriptions from the final
api-documentation PR. The other part is just my messing with stuff to
try to make it look better.

(also, move image_optimizer_default_settings guide from docs to templates)

…tion.

This is working well! Just wanted to add this to double-confirm it's
doing what I expect.

Not sure why updating had caused this, but this should still work.