Update ccall documentation

[ViralBShah]

Update ccall documentation to capture the new functionality in commit 2ba6116

1. Conversions are inserted automatically, so you can simply pass "x,
   y, z" instead of the "int32(x), uint64(y), int32(z)" we had to use
   everywhere. Of course, this uses the convert() function. Arrays and
   byte strings can be converted to pointers.
2. A pointer to a scalar can be passed using the syntax "&x". This is
   especially useful for calling fortran. This syntax is a pun and does
   not do the same thing as C. In particular, while it is safe for called
   functions to write through this pointer, you will not be able to see
   its changes (the variable "x" will not change). Things like &0 and
   &f(x) also work, since this is not a real address-of operator.
   Conversion also happens with &; passing &x as a Ptr{T} will first
   convert x to type T.

[JeffBezanson]

Done.