docs(ByAltText): Add notes about support for custom elements (#946)

Co-authored-by: Tim Deschryver <[email protected]>

Author: eps1lon

docs/queries/about.mdx

@@ -124,7 +124,7 @@ possible. With this in mind, we recommend this order of priority:
    experience of interacting with these attributes varies greatly across
    browsers and assistive technology.
    1. `getByAltText`: If your element is one which supports `alt` text (`img`,
-      `area`, and `input`), then you can use this to find that element.
+      `area`, `input`, and any custom element), then you can use this to find that element.
    1. `getByTitle`: The title attribute is not consistently read by
       screenreaders, and is not visible by default for sighted users
 1. **Test IDs**

docs/queries/byalttext.mdx

@@ -23,7 +23,7 @@ getByAltText(
 ```
 
 This will return the element (normally an `<img>`) that has the given `alt`
-text. Note that it only supports elements which accept an `alt` attribute:
+text. Note that it only supports elements which accept an `alt` attribute or [custom elements](https://developer.mozilla.org/en-US/docs/Web/Web_Components/Using_custom_elements) (since we don't know if a custom element implements `alt` or not):
 [`<img>`](https://developer.mozilla.org/en-US/docs/Web/HTML/Element/img),
 [`<input>`](https://developer.mozilla.org/en-US/docs/Web/HTML/Element/input),
 and [`<area>`](https://developer.mozilla.org/en-US/docs/Web/HTML/Element/area)