Improve :has() pseudo-class documentation

[dipikabh]

Description

This PR improves the documentation of the :has() page and updates the documentation of a few other pages where related terms appear. Here's a summary of these changes:

1. :has() pseudo-class page (https://developer.mozilla.org/en-US/docs/Web/CSS/:has)

• Improves the description and adds link to the definition of the term "forgiving selector list"
• Adds three examples to showcase different scenarios for using :has()

2. CSS selectors page (https://developer.mozilla.org/en-US/docs/Web/CSS/CSS_Selectors)

• Improves the formatting on the page so that "Syntax" and "Examples" for the different type of selectors appear on newlines
• Adds the section "## Structure of a selector" to define the terms simple selectors, compound selectors, complex selectors, relative selectors, and selector lists

3. Pseudo-class page (https://developer.mozilla.org/en-US/docs/Web/CSS/Pseudo-classes) - includes minor language improvements

4. Selector list page (https://developer.mozilla.org/en-US/docs/Web/CSS/Selector_list)

• Explains the terms "forgiving selector list" and "forgiving relative selector list"

Additional details

https://drafts.csswg.org/selectors/#relational

Related issues and pull requests

Fixes #19972

[github-actions[bot]]

Preview URLs

• /en-US/docs/Web/CSS/:has
• /en-US/docs/Web/CSS/CSS_Selectors
• /en-US/docs/Web/CSS/Pseudo-classes
• /en-US/docs/Web/CSS/Selector_list

Flaws (8)

Note! 2 documents with no flaws that don't need to be listed. 🎉

URL: /en-US/docs/Web/CSS/Pseudo-classes Title: Pseudo-classes Flaw count: 7

• macros:
  • /en-US/docs/Web/CSS/:dir() redirects to /en-US/docs/Web/CSS/:dir
  • /en-US/docs/Web/CSS/:lang() redirects to /en-US/docs/Web/CSS/:lang
  • wrong xref macro used (consider changing which macro you use)
  • wrong xref macro used (consider changing which macro you use)
  • wrong xref macro used (consider changing which macro you use)
  • and 2 more flaws omitted

---

URL: /en-US/docs/Web/CSS/CSS_Selectors Title: CSS selectors Flaw count: 1

• macros:
  • /en-US/docs/Web/HTML/Element/h2 redirects to /en-US/docs/Web/HTML/Element/Heading_Elements

External URLs (1)

URL: /en-US/docs/Web/CSS/:has Title: :has()

• https://drafts.csswg.org/selectors-4/ (1 time) (Note! This may be a new URL 👀)

(this comment was updated 2022-10-04 16:27:26.276179)

[dipikabh]

Hi @estelle, we can probably land this now and make more iterations in future PRs. Thank you!