doc: clarify sourceMap.payload behavior and immutability

Author: watilde

doc/api/module.md

@@ -1970,6 +1970,9 @@ generated code.
 
 Getter for the payload used to construct the [`SourceMap`][] instance.
 
+The returned object is frozen with [`Object.freeze()`][], and the same
+reference is returned on every access. Do not mutate the returned object.
+
 #### `sourceMap.findEntry(lineOffset, columnOffset)`
 
 * `lineOffset` {number} The zero-indexed line number offset in
@@ -2065,6 +2068,7 @@ returned object contains the following keys:
 [`module`]: #the-module-object
 [`os.tmpdir()`]: os.md#ostmpdir
 [`register`]: #moduleregisterspecifier-parenturl-options
+[`Object.freeze()`]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object/freeze
 [`util.TextDecoder`]: util.md#class-utiltextdecoder
 [accepted final formats]: #accepted-final-formats-returned-by-load
 [asynchronous `load` hook]: #asynchronous-loadurl-context-nextload