doc scm_c_read, scm_c_write

[Kevin Ryde]

A couple of bits from the news file,

        * scheme-io.texi (Reading): Add scm_c_read.
        (Writing): Add scm_c_write.

 - C Function: size_t scm_c_read (SCM port, void *buffer, size_t size)
     Read up to SIZE bytes from PORT and store them in BUFFER.  The
     return value is the number of bytes actually read, which can be
     less than SIZE, and indeed possibly zero, if end-of-file has been
     reached.

     Note that this function does not update `port-line' and
     `port-column' above.

 - C Function: void scm_c_write (SCM port, const void *buffer, size_t
          size)
     Write SIZE bytes from BUFFER to PORT.

     Note that this function does not update `port-line' and
     `port-column' (*note Reading::).