Update TT_New_Context documentation

In 531d463a "[truetype] Allocate TT_ExecContext in TT_Size instead of TT_Driver." the TT_ExecContext was moved from being on the driver to being on the size to make it easier to use FreeType in a multi-threaded environment. However, the documentation for TT_New_Context was not updated and still reflects the old behavior and parameter list.

This change updates TT_New_Context documentation to reflect the current parameters and usage.