Fix inaccurate wording about cubic-bezier validity

[MUSKANNISHAD]

This PR clarifies the wording around the validity of cubic Bézier curves used as easing functions.

It removes an overly strong “if and only if” claim and aligns the explanation with the CSS specification, while keeping the behavior description intact.

Fixes #42526

[estelle]

Thanks! 🎉

[github-actions]

Preview URLs

• /en-US/docs/Web/CSS/Reference/Values/easing-function/cubic-bezier

Flaws (6)

URL: /en-US/docs/Web/CSS/Reference/Values/easing-function/cubic-bezier
Title: cubic-bezier()
Flaw count: 6

• broken_links:
  • Link /en-US/docs/Web/CSS/CSS_values_and_units/Value_definition_syntax#brackets is a redirect
  • Link /en-US/docs/Web/CSS/number is a redirect
  • Link /en-US/docs/Web/CSS/number is a redirect
  • Link /en-US/docs/Web/CSS/CSS_values_and_units/Value_definition_syntax#brackets is a redirect
  • Link /en-US/docs/Web/CSS/CSS_values_and_units/Value_definition_syntax#hash_mark is a redirect
  • and 1 more flaws omitted

External URLs (1)

URL: /en-US/docs/Web/CSS/Reference/Values/easing-function/cubic-bezier
Title: cubic-bezier()

• https://en.wikipedia.org/wiki/Function_%28mathematics%29 (1 time) (Note! This may be a new URL 👀)