MFC r364824:

r364824 | imp | 2020-08-26 11:06:16 -0600 (Wed, 26 Aug 2020) | 8 lines

Make sbuf_setpos match the implementation.

sbuf_setpos can only be used to truncate the buffer, never to make it
longer. Update the documentation to reflect this.

Reviewed By: allanjude, phk
Differential Revision: https://reviews.freebsd.org/D26198