Docs: Clarify whitespace behavior in textWidth() (Fixes #7745)

[AsthaMishra-ash]

Resolves #7745

Changes:

• Clarified the documentation of textWidth() to explain how whitespace characters are handled.
• No functional code changes were made.
• This is a documentation-only update.

Screenshots of the change:

Not applicable (documentation update only).

PR Checklist

• npm run lint passes
• [Inline reference] is included / updated
• [Unit tests] are included / updated

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