On the man page mention -readable by whom

[積丹尼 Dan Jacobson]

$ man find

       -readable
              Matches files which are readable.  This takes into account access 
 control  lists  and  other  permissions
              artefacts  which  the -perm test ignores.  This test makes use of 
the access(2) system call, and so can be
              fooled by NFS servers which do UID mapping (or root-squashing), 
since many systems implement access(2)  in
              the client's kernel and so cannot make use of the UID mapping 
information held on the server.

Mention readable by whom, so one need not check the Info manual.