OPENNLP-1791: Document the use of ClassPathModelProvider in dev manual

[mawiesne]

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[rzo1]

Should we mention adding
--add-opens java.base/jdk.internal.loader=ALL-UNNAMED
as a JVM argument here? Otherwise, it may not be clear to users that OpenNLP will fall back to the bootstrap class path for model lookup instead of using the UCP class loader

Maybe something like:

Note: When running on Java 17+, the JVM argument
--add-opens java.base/jdk.internal.loader=ALL-UNNAMED
may be required. Without this option, OpenNLP uses the JVM bootstrap class path to locate models rather than the UCP class loader. 

For more advanced or non-standard class loading scenarios, using ClassGraph and implementing a custom provider may cover additional cases beyond the default UCP class loader or JVM bootstrap class path behavior.

[mawiesne]

@rzo1: Most recent commit:

• adds note on JVM argument since J17 and beyond
• improves formatting of all "Note:" text fragments to better highlight helpful context to the reader