Document use of wildcards (*) and ampersands (&) in autofs map files.

Differential Revision:	https://reviews.freebsd.org/D1199
Reviewed by:	wblock@
MFC after:	1 month
Sponsored by:	The FreeBSD Foundation
Fix error handling.

MFC after:	1 month
Coverity CID:	1249768
Sponsored by:	The FreeBSD Foundation