Explain the typographic rationale for text-autospace - #46251

Fixes #46247

Summary

• explain why mixed CJK and Latin or numeric text may need automatic spacing
• clarify that text-autospace avoids adding whitespace characters to source content
• link the relevant W3C Chinese and Japanese layout guidance for further reading

Validation

• Prettier check for the edited page
• Markdownlint for the edited page
• CSpell for the edited page
• MDN filecheck for the edited page
• repository pre-commit checks (front matter, Prettier, Markdownlint, xrefs, and URLs)