Fix Extract documentation, and possible fix implementations

[bogdandrutu]

Describe the bug
The specs says:
https://github.com/open-telemetry/opentelemetry-specification/blob/main/specification/context/api-propagators.md#extract

Our documentation says:
https://github.com/open-telemetry/opentelemetry-java/blob/main/api/context/src/main/java/io/opentelemetry/context/propagation/TextMapPropagator.java#L113

Steps to reproduce
If possible, provide a recipe for reproducing the error.

What did you expect to see?
A contract compatible with the specification.

What did you see instead?
A contract incompatible with the specification.

[bogdandrutu]

Documentation was updated, the verification/implementation is tracked in #2933