OPENNLP-287: Extend POS Tagger documentation with more information about the tag dictionary

[NishantShri4]

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[NishantShri4]

Post changes, the rendered documentation (for Tag Dictionary section) looks like this -
PosTagger_Tag_Dictionary_Documentation_After_Changes_PR_754.pdf

[NishantShri4]

Hi @kottmann - Would you like to review this one and share comments?

[jzonthemtn]

Thanks @NishantShri4! Definitely a gap in the documentation. This looks good to me. I admit I did not run the example code though.

[mawiesne]

Thx a lot @NishantShri4 for your first contribution to Apache OpenNLP! Highly welcome and a valuable addition to the dev manual! I've projected that issue for the upcoming maintenance release 2.5.4

[atarora]

Looks good ! Thank you !