Clarify that non-aws-config based config Builders don't have good defaults

[Mark-Simulacrum]

Describe the issue

Customers using e.g. aws_sdk_sts::config::Builder::new() and customizing a few settings (e.g., credential provider) don't currently receive defaults for other fields, such as the retry policy.

Ideally we'd have strong warnings against usage of these methods in favor of the aws-config constructors, and potentially consider some stronger messaging (e.g., deprecating in favor of without_defaults() or similarly less "easy to use" methods). Note that there's a bunch of ways to get such a builder today:

• https://docs.rs/aws-sdk-sts/latest/aws_sdk_sts/config/struct.Builder.html#method.new
• https://docs.rs/aws-sdk-sts/latest/aws_sdk_sts/config/struct.Config.html#method.builder

Each should likely have similar treatment. The docs on the Config struct also say:

The service config can also be constructed manually using its builder.

Which probably should be clarified to say "and this is almost certainly not what you want".

Links

(Also present in every other SDK):

• https://docs.rs/aws-sdk-sts/latest/aws_sdk_sts/config/struct.Builder.html#method.new
• https://docs.rs/aws-sdk-sts/latest/aws_sdk_sts/config/struct.Config.html#method.builder