Allow for strings in iex h helper
Add h helper doc for string arguments
Rephrase h helper doc for string arguments
Use list argument in h helper for strings

This will check the function name in three different places (__MODULE__, Kernel, and Kernel.SpecialForms), allowing one to ask the docs for the new operator &.
Merge pull request #1542 from thmzlt/iex-h-helper-string

Allow for strings in iex h helper