docs: clarify FlatList extraData prop and PureComponent behavior (fixes #5216)

[supunyasanthaofficial]

Summary

Fixes #5216.

Clarifies how extraData works in the selectable FlatList example in docs/flatlist.md. The change is also applied to the versioned docs from 0.77 through 0.87.

Changes

• Explained that extraData tells FlatList when it needs to account for changes in data outside its data prop.
• Clarified that FlatList uses shallow prop comparison, so extraData is important when renderItem doesn't change its reference but depends on external state.
• Removed the ambiguous use of "we" to follow the documentation style guide.

Test Plan

• Ran npx eslint docs/flatlist.md with no errors.
• Checked formatting across the versioned docs from 0.77 through 0.87.

[theprantadutta]

I'd looked into #5216 earlier, so I checked this against the source: FlatList extends React.PureComponent (FlatList.js). So with a stable renderItem and unchanged data, the shallow prop check skips the re-render entirely, and extraData is what gets it through. The wording here is right.

It also corrects something in my comment on the issue: I tied the stable-renderItem case to strictMode, which was too narrow. strictMode just adds a second memoization layer on top, and the PureComponent check applies either way.

This is from reading the source, not a device run.