helper/schema: Additional Go documentation for Resource and Schema types (#912)

Reference: hashicorp/terraform-plugin-sdk#467
Reference: hashicorp/terraform-plugin-sdk#601
Reference: hashicorp/terraform-plugin-sdk#705
Reference: hashicorp/terraform-plugin-sdk#735

This also spends some cycles fixing the Go documentation for struct fields to be aligned with each field individually, so pkg.go.dev and the Go language server can appropriately show the matching documentation.

Author: bflad