trivial: document ext3 semantics of 'ro' option a bit better

ext3 has quite unexpected semantics or "ro" and defaults are
not what they are documented to be, due to mkfs override.

Signed-off-by: Pavel Machek <pavel@suse.cz>
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>