staging: comedi: comedidev.h: tweak kernel-doc for struct comedi_async

 *

 * Think of the _Count values being integers of unlimited size, indexing

 * into a buffer of infinite length (though only an advancing portion

 * of the buffer of fixed length prealloc_bufsz is accessible at any time).

 * Then:

 * of the buffer of fixed length prealloc_bufsz is accessible at any

 * time).  Then:

 *

 *   Buf_Read_Count <= Buf_Read_Alloc_Count <= Munge_Count <=

 *   Buf_Write_Count <= Buf_Write_Alloc_Count <=

 *   (Buf_Read_Count + prealloc_bufsz)

 *

 * (Those aren't the actual members, apart from prealloc_bufsz.) When

 * the buffer is reset, those _Count values start at 0 and only increase

 * in value, maintaining the above inequalities until the next time the

 * buffer is reset.  The buffer is divided into the following regions by

 * the inequalities:

 * (Those aren't the actual members, apart from prealloc_bufsz.) When the

 * buffer is reset, those _Count values start at 0 and only increase in value,

 * maintaining the above inequalities until the next time the buffer is

 * reset.  The buffer is divided into the following regions by the inequalities:

 *

 *   [0, Buf_Read_Count):

 *     old region no longer accessible

 *

 *   [Buf_Read_Count, Buf_Read_Alloc_Count):

 *     filled and munged region allocated for reading but not yet read

 *

 *   [Buf_Read_Alloc_Count, Munge_Count):

 *     filled and munged region not yet allocated for reading

 *

 *   [Munge_Count, Buf_Write_Count):

 *     filled region not yet munged

 *

 *   [Buf_Write_Count, Buf_Write_Alloc_Count):

 *     unfilled region allocated for writing but not yet written

 *

 *   [Buf_Write_Alloc_Count, Buf_Read_Count + prealloc_bufsz):

 *     unfilled region not yet allocated for writing

 *

 *   [Buf_Read_Count + prealloc_bufsz, infinity):

 *     unfilled region not yet accessible

 *