Changing api documentation for jsx transformer

Hi in chrome debugger, the files that use sourcemaps from the transformer get "null" as the filename during debug, https://github.com/facebook/react/blob/master/main.js#L15 "sourceFilename" is used instead of "filename" to specify the filename in the sourcemap.
Merge pull request #2413 from LeonFedotov/patch-1

Changing api documentation for jsx transformer