landlock: Add design choices documentation for filesystem access rights

Summarize the rationale of filesystem access rights according to the
file type.

Update the document date.

Reviewed-by: Paul Moore <[e-mail]>
Signed-off-by: Mickaël Salaün <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]

Documentation/security/landlock.rst

···
 ==================================
 
 :Author: Mickaël Salaün
-:Date: March 2021
+:Date: May 2022
 
 Landlock's goal is to create scoped access-control (i.e. sandboxing).  To
 harden a whole system, this feature should be available to any process,
···
   processes.
 * Computation related to Landlock operations (e.g. enforcing a ruleset) shall
   only impact the processes requesting them.
+
+Design choices
+==============
+
+Filesystem access rights
+------------------------
+
+All access rights are tied to an inode and what can be accessed through it.
+Reading the content of a directory doesn't imply to be allowed to read the
+content of a listed inode.  Indeed, a file name is local to its parent
+directory, and an inode can be referenced by multiple file names thanks to
+(hard) links.  Being able to unlink a file only has a direct impact on the
+directory, not the unlinked inode.  This is the reason why
+`LANDLOCK_ACCESS_FS_REMOVE_FILE` or `LANDLOCK_ACCESS_FS_REFER` are not allowed
+to be tied to files but only to directories.
 
 Tests
 =====