Document additional metadata values in UA

Author: dagnir

docs/user-agent.md

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+# User Agent
+
+## Additional Metadata
+
+The table below documents additional metadata the SDK may include in the `User-Agent` header of a request. These take the form of `md/[name]#[value]` in the request.
+
+|Name|Value(s)|Description|
+|---|---|---|
+|`rb`||The request body implementation. This includes `ContentStreamProvider` for sync clients, and `AsyncRequestBody` for async clients.|
+||`f`|"File". The body implementation reads from a file.|
+||`b`|"Bytes". The body implementation reads from a byte array.|
+||`c`|"String" The body implementation reads from a string. |
+||`s`|"Stream". The body implementation reads from an `InputStream`.|
+||`p`|"Publisher". The body implementation reads from an `SdkPublisher`.|
+||`u`|"unknown"|
+|`rt`||The response transformer implementation. This includes `ResponseTransformer` for sync clients, and `AsyncResponseTransformer` for async clients.|
+||`f`|"File". The response transformer writes the response body to a file.|
+||`b`|"Bytes". The response transformer writes the response body to a byte array.|
+||`s`|"Stream". The response transformer adapts the response body to an `InputStream`.|
+||`p`|"Publisher". The response transformer adapts the response body to an `SdkPublisher`.|
+||`u`|"unknown"|