Update Javadoc about normalizeEntry in Kuromoji DictionaryBuilder [LUCENE-9270]

The normalizeEntry option is missing from the Javadoc of Kuromoji DictionaryBuilder.
Without this explanation, users don't know what it means until they see the code.
Also, if user follows the usage of Javadoc, it will not be built.

So the following contents need to be applied:

1) Change usage
before:
  java -cp [lucene classpath] org.apache.lucene.analysis.ja.util.DictionaryBuilder \
    ${inputDir} ${outputDir} ${encoding}
after:
  java -cp [lucene classpath] org.apache.lucene.analysis.ja.util.DictionaryBuilder \
    ${inputDir} ${outputDir} ${encoding} **${normalizeEntry}**

2) Add description about normalizeEntry

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Migrated from [LUCENE-9270](https://issues.apache.org/jira/browse/LUCENE-9270) by Namgyu Kim (@danmuzi), updated Mar 11 2020