docs(text-style-props): clarify textAlign behavior under RTL

[gor3a]

Summary

The textAlign entry only lists the enum values with no explanation of how they behave under a right-to-left layout direction. This is a recurring source of confusion (e.g. developers expecting textAlign: 'right' to render visually right in RTL).

This adds a short note clarifying that:

• the physical values 'left'/'right' are mirrored under RTL ('left' renders on the visual right, so it acts as the start edge),
• 'center'/'justify' are not mirrored,
• there is no 'start'/'end' value yet (links the tracking issue #45255).

Docs only — no behavior change.

Context

Motivated by RTL textAlign confusion reported in expo/expo#39752 — see this comment for the full root-cause breakdown of how textAlign behaves under an RTL layout direction.

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