Provide a link to MDN for options and their values (#768)

An implementer contacted me asking where the specific options and their values were documented. I pointed out the documentation in MDN.

This PR provides non-normative links to that information to help others.

Author: aphillips

spec/registry.md

@@ -367,6 +367,11 @@ The defaults for these options are implementation-dependent.
 In general, the default values for such options depend on the locale, 
 the value of other options, or both.
 
+> [!NOTE]
+> The names of _options_ and their _values_ were derived from the
+> [options](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Intl/NumberFormat/NumberFormat#options)
+> in JavaScript's `Intl.NumberFormat`.
+
 The following options and their values are required to be available on the function `:number`:
 - `select`
    -  `plural` (default; see [Default Value of `select` Option](#default-value-of-select-option) below)
@@ -492,6 +497,11 @@ The defaults for these options are implementation-dependent.
 In general, the default values for such options depend on the locale, 
 the value of other options, or both.
 
+> [!NOTE]
+> The names of _options_ and their _values_ were derived from the
+> [options](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Intl/NumberFormat/NumberFormat#options)
+> in JavaScript's `Intl.NumberFormat`.
+
 The following options and their values are required in the default registry to be available on the 
 function `:integer`:
 - `select`
@@ -777,6 +787,11 @@ output.
 If both are specified, an _Invalid Expression_ error MUST be emitted
 and a _fallback value_ used as the resolved value of the _expression_.
 
+> [!NOTE]
+> The names of _options_ and their _values_ were derived from the
+> [options](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Intl/DateTimeFormat/resolvedOptions#description)
+> in JavaScript's `Intl.DateTimeFormat`.
+
 ##### Style Options
 
 The function `:datetime` has these _style options_.