mdadm: Update config man regarding default files and multi-keyword behavior

mdadm: Update config man regarding default files and multi-keyword behavior

Simplify default and alternative config file and directory location references
from mdadm(8) as references to mdadm.conf(5). Add FILE section in config man
and explain order and conditions in which default and alternative config files
and directories are used.

Update config man behavior regarding parsing order when multiple keywords/config
files are involved.

Signed-off-by: Lukasz Florczak <lukasz.florczak@linux.intel.com>
Acked-by: Coly Li <colyli@suse.de>
Signed-off-by: Jes Sorensen <jsorensen@fb.com>