Clarify attr() URL limitations and add image-set() example

[codersjj]

Description

This updates the explanation of how attr() behaves when used in URL-related contexts.
The previous text implied a general restriction, but the real limitation applies only when the value is required to be a <url> type.

The revised wording explains that:

• attr() values are attr-tainted.
• Using them in a context requiring a <url> (e.g. inside url() or background-image) results in IACVT.
• Some functions—such as image-set()—accept a <string> first, which is later interpreted as a URL. This can allow attr() to work in shorthand properties like background, although it does not work in background-image.

Motivation

This update was made after watching a video by zhangxinxu on bilibili:
https://www.bilibili.com/video/BV1dPkzB3Esx
I realized the documentation should more accurately reflect the difference between <url>-required contexts and cases where <string> is allowed before URL resolution.

[github-actions]

Preview URLs

• /en-US/docs/Web/CSS/Reference/Values/attr

Flaws (52)

URL: /en-US/docs/Web/CSS/Reference/Values/attr
Title: attr()
Flaw count: 52

• broken_links:
  • /en-US/docs/Web/CSS/CSS_values_and_units/Value_definition_syntax#question_mark is a redirect
  • Can't resolve /en-US/docs/Web/CSS/declaration-value
  • /en-US/docs/Web/CSS/CSS_values_and_units/Value_definition_syntax#question_mark is a redirect
  • /en-US/docs/Web/CSS/CSS_values_and_units/Value_definition_syntax#brackets is a redirect
  • Can't resolve /en-US/docs/Web/CSS/ident-token
  • and 32 more flaws omitted
• macros:
  • Macro produces link /en-US/docs/Web/CSS/content which is a redirect
  • Macro produces link /en-US/docs/Web/CSS/var which is a redirect
  • Macro produces link /en-US/docs/Web/CSS/type which is a redirect
  • Macro produces link /en-US/docs/Web/CSS/type which is a redirect
  • Macro produces link /en-US/docs/Web/CSS/url_value which is a redirect
  • and 10 more flaws omitted