Improve PayloadConverter javadoc regarding providing of EncodingKeys#METADATA_ENCODING_KEY#702

Spikhalskiy merged 2 commits intotemporalio:mastertemporalio/sdk-java:masterfrom

Spikhalskiy:payloadconverter-javadocSpikhalskiy/java-sdk:payloadconverter-javadocCopy head branch name to clipboard