Improve SDK docs around support for modeled request compression

[lucix-aws]

See #2753. There is a discoverability issue with the modeled request compression feature in that there's no explicit indication of what services actually support it. The presence of the env settings for controlling it are rife for causing confusion as customers might be lead to believe there is functionality where none exists.

Some things we can do better:

• Clarify documentation in the SDK around these settings to clarify the per-request nature
• Add custom doc strings to operations that support it indicating as such