Clarified the docs on string concatenation (see #664)
Clarified the docs on string concatenation (see #664)

FSMaxB/mathjs,FSMaxB/mathjs,FSMaxB/mathjs,josdejong/mathjs,josdejong/mathjs,josdejong/mathjs,FSMaxB/mathjs,FSMaxB/mathjs,josdejong/mathjs,josdejong/mathjs