Convert zfs_mg_noalloc_threshold to a module parameter and document

The parameter was added as illumos issue 4081 which was committed to
zfsonlinux in ac72fac3eaa569902cad88053167f7d74e7fe7e4. This patch
documents the parameter and allows for it to be set as a module parameter.

Signed-off-by: Tim Chase <tim@chase2k.com>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #2483