Docs/admin-guide/mm/damon/usage: document DAMON sysfs interface

Docs/admin-guide/mm/damon/usage: document DAMON sysfs interface

This commit adds detailed usage of DAMON sysfs interface in the
admin-guide document for DAMON.

Link: https://lkml.kernel.org/r/[email protected]
Signed-off-by: SeongJae Park <[email protected]>
Cc: David Rientjes <[email protected]>
Cc: Greg Kroah-Hartman <[email protected]>
Cc: Jonathan Corbet <[email protected]>
Cc: Shuah Khan <[email protected]>
Cc: Xin Hao <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>