So I played around with this a lot more and figured some things out. The curved right arrow does indeed make it learn the word. Type something strange, tap the curved arrow and then type the same thing again and you will see your new word in the list. Also, long press the new word and it will give you the option to forget it.
I also figured that all this would be specific to whatever keyboard you are using so explore the options under Samsung Keyboard and you will find many interesting things including these predictive text settings:
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