[css-shadow-parts] make clear that Shadow Parts for built-in elements should not be supported without standardization

https://drafts.csswg.org/css-shadow-parts/

The ::part() pseudo-element looks to be a natural syntax to use to expose access to bits of built-in HTML form controls, such as the button in an <input type=file>. We at Mozilla have a concern that this syntax will be used to expose parts of built-in elements (i.e. not custom element defined elements) without going through the standardization process. We have seen in the past that engines have readily exposed ::-webkit-* and ::-moz-* pseudo elements so that authors can style bits of built-in elements. Solving the HTML form element stylability problem is a big task, and we would like to avoid vendors exposing their built-in element internals through ::part() without discussion in an appropriate standards group. We request that a note be added to the spec clarifying that ::part() is not to be used as a general implementation defined extension mechanism, and that any exposure of built-in element parts should be done by going through the regular standardization process.

Start with the linked CSS Shadow Parts draft, especially the section defining the ::part() pseudo-element and its intended scope. Add a note clarifying that built-in element parts should not be exposed through ::part() without regular standardization, then verify the wording fits the surrounding specification text.