[docs][Selects] update docs to reflect the omission of placeholder pr…

…op to the select component

docs/data/material/components/selects/selects.md

@@ -34,6 +34,10 @@ The Select component is implemented as a custom `<input>` element of the [InputB
 It extends the [text field components](/material-ui/react-text-field/) subcomponents, either the [OutlinedInput](/material-ui/api/outlined-input/), [Input](/material-ui/api/input/), or [FilledInput](/material-ui/api/filled-input/), depending on the variant selected.
 It shares the same styles and many of the same props. Refer to the respective component's API page for details.
 
+:::warning
+The `placeholder` prop is NOT shared.
+:::
+
 ### Filled and standard variants
 
 {{"demo": "SelectVariants.js"}}