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
@@ -2054,6 +2057,7 @@ returned object contains the following keys:
 [`NODE_COMPILE_CACHE_PORTABLE=1`]: cli.md#node_compile_cache_portable1
 [`NODE_DISABLE_COMPILE_CACHE=1`]: cli.md#node_disable_compile_cache1
 [`NODE_V8_COVERAGE=dir`]: cli.md#node_v8_coveragedir
+[`Object.freeze()`]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object/freeze
 [`SourceMap`]: #class-modulesourcemap
 [`initialize`]: #initialize
 [`module.constants.compileCacheStatus`]: #moduleconstantscompilecachestatus