Documentation/dax: Update description of DAX policy changing

After commit 77573fa3 ("fs: Kill DCACHE_DONTCACHE dentry even if
DCACHE_REFERENCED is set"), changes to DAX policy will take effect
as soon as all references to this file are gone.

Update the documentation accordingly.

Signed-off-by: Hao Li <lihao2018.fnst@cn.fujitsu.com>
Reviewed-by: Ira Weiny <ira.weiny@intel.com>
Link: https://lore.kernel.org/r/20210106015000.5263-1-lihao2018.fnst@cn.fujitsu.com


Signed-off-by: Jonathan Corbet <corbet@lwn.net>