This has been fixed in Precise (if not earlier): libsane depends on libsane-common, which now provides html/sane-mfgs.html.

jani@saegusa:~$ dlocate sane-mfgs.html
libsane-common: /usr/share/doc/libsane/html/sane-mfgs.html
jani@saegusa:~$ LC_ALL=C apt-cache depends libsane | grep libsane
libsane
Depends: libsane-common
libsane-common:i386
Suggests: libsane-extras
Replaces: libsane-extras
Replaces: libsane-extras:i386
Replaces: libsane:i386
Breaks: libsane:i386