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.