Add and adjust alpha parameter description

[FrauBoes]

Addresses #657

Changes:

1. Remove the word 'optional' from the description of the alpha parameter of fill()
2. Add description of alpha parameter of stroke()

(1) is done because '(Optional)' is added to the description automatically.
(2) is done because the description was missing.

Screenshots of the change:

PR Checklist

• npm run lint passes
• Inline documentation is included / updated
• Unit tests are included / updated

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