fix(mix-blend-mode): replace "parent and background" with "backdrop" in description

[xonaib]

Fixes #41078

The current description says the element blends with "the content of the element's parent
and the element's background", which has two problems:

1. "element's background" implies the CSS background property of the element itself.
   That is what background-blend-mode does. mix-blend-mode does NOT blend with the
   element's own background.

2. "element's parent" is imprecise. Blending happens against the backdrop — all rendered
   content behind the element within the same stacking context — which may include siblings,
   not just the parent.

Per the W3C CSS Compositing and Blending spec §3.4.1:

"An element that has blending applied, must blend with all the underlying content of
the stacking context that that element belongs to."

Changes:

• Replaced the inaccurate description with "backdrop" (the spec's term), linked to
  the stacking context guide
• Added a Note callout clarifying the distinction from background-blend-mode

Verified locally:

• markdownlint-cli2 files/en-us/web/css/reference/properties/mix-blend-mode/index.md
• node scripts/filecheck/index.js --cwd=. files/en-us/web/css/reference/properties/mix-blend-mode/index.md
• MDN pre-commit checks: changed xrefs, front matter, Prettier, Markdownlint