Improve Javadoc for Coder class and clarify Context usage #42948

Job	Run time
Java Wordcount Direct Runner (windows-latest)	22m 45s
Java Wordcount Direct Runner (macos-latest)	19m 22s
Java Unit Tests (windows-latest)	18m 2s
Java Unit Tests (macos-latest)	12m 26s
Java Unit Tests (self-hosted, ubuntu-24.04, main)	18m 17s
Java Wordcount Direct Runner (self-hosted, ubuntu-24.04, main)	10m 2s
	1h 40m 54s

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