update SMB_BWRITE documentation, clarify SMB_BREAD

After removal of SMB_TRANS some information in the description of
SMB_BWRITE has become stale. E.g., the maximum block size has been
restored to 32.

Also, the descriptions of SMB_BREAD and SMB_BWRITE had some
incorrect information on the SMBus protocol details.

MFC after: 1 week
X-MFC with: r308242
Differential Revision: https://reviews.freebsd.org/D8431