Update fragment's definition in Learn-Queries.md

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Updating the wording on how fragment should be used to nudge people towards a more scalable approach of using fragments to define data requirements.

Description

Even though the spec allows fragments to be used however people see fit, the document should nudge people towards a better usage pattern that scales well. In this case, attempt to describe them as composable pieces of data requirements used by UI or utility code. I hope this isn't controversial.

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@benjie thank you for the review! I can do a follow PR to rename the fragment across the document.