Add docs for #15340

border-radius CSS property doesn't affect <select>s in some browsers.
Hence, @input-border-radius doesn't affect <select class="form-control"> in some browsers.
Closes #15340.

[skip sauce]
[skip validator]

less/variables.less

@@ -187,6 +187,7 @@
 
 // TODO: Rename `@input-border-radius` to `@input-border-radius-base` in v4
 //** Default `.form-control` border radius
+// This has no effect on `<select>`s in some browsers, due to the limited stylability of `<select>`s in CSS.
 @input-border-radius:            @border-radius-base;
 //** Large `.form-control` border radius
 @input-border-radius-large:      @border-radius-large;