Document g_reset_bio(). This is long overdue. g_reset_bio will reset
the bio to a pristine state should you wish to re-use it for another
I/O without freeing it. In the bast, a simple bzero was done to do
this, but that may not be sufficient in the future when the bio may
contain state that's not part of the documented API. Besides, it makes
the code clearer as to the intent...

Noticed by: smh@