Add note about generating source maps to load() and transform() (#3252)

* mention `map` property in `parse` response

* add note about `map` in return from `load`

* remove empty line

* fix typo

docs/05-plugin-development.md

@@ -173,7 +173,7 @@ Type: `(id: string) => string | null | { code: string, map?: string | SourceMap,
 Kind: `async, first`<br>
 Phase: `build`
 
-Defines a custom loader. Returning `null` defers to other `load` functions (and eventually the default behavior of loading from the file system). To prevent additional parsing overhead in case e.g. this hook already used `this.parse` to generate an AST for some reason, this hook can optionally return a `{ code, ast }` object. The `ast` must be a standard ESTree AST with `start` and `end` properties for each node.
+Defines a custom loader. Returning `null` defers to other `load` functions (and eventually the default behavior of loading from the file system). To prevent additional parsing overhead in case e.g. this hook already used `this.parse` to generate an AST for some reason, this hook can optionally return a `{ code, ast, map }` object. The `ast` must be a standard ESTree AST with `start` and `end` properties for each node. If the transformation does not move code, you can preserve existing sourcemaps by setting `map` to `null`. Otherwise you might need to generate the source map. See [the section on source code transformations](#source-code-transformations).
 
 If `false` is returned for `moduleSideEffects` and no other module imports anything from this module, then this module will not be included in the bundle without checking for actual side-effects inside the module. If `true` is returned, Rollup will use its default algorithm to include all statements in the module that have side-effects (such as modifying a global or exported variable). If `null` is returned or the flag is omitted, then `moduleSideEffects` will be determined by the first `resolveId` hook that resolved this module, the `treeshake.moduleSideEffects` option, or eventually default to `true`. The `transform` hook can override this.
 
@@ -317,7 +317,7 @@ Type: `(code: string, id: string) => string | null | { code: string, map?: strin
 Kind: `async, sequential`<br>
 Phase: `build`
 
-Can be used to transform individual modules. To prevent additional parsing overhead in case e.g. this hook already used `this.parse` to generate an AST for some reason, this hook can optionally return a `{ code, ast }` object. The `ast` must be a standard ESTree AST with `start` and `end` properties for each node.
+Can be used to transform individual modules. To prevent additional parsing overhead in case e.g. this hook already used `this.parse` to generate an AST for some reason, this hook can optionally return a `{ code, ast, map }` object. The `ast` must be a standard ESTree AST with `start` and `end` properties for each node. If the transformation does not move code, you can preserve existing sourcemaps by setting `map` to `null`. Otherwise you might need to generate the source map. See [the section on source code transformations](#source-code-transformations).
 
 Note that in watch mode, the result of this hook is cached when rebuilding and the hook is only triggered again for a module `id` if either the `code` of the module has changed or a file has changed that was added via `this.addWatchFile` the last time the hook was triggered for this module.