init doc: --shared=0xxx does not give umask but perm bits

The description that 0640 makes sure that the group members can read
the repository is correct, but calling that octal number a <umask>
is wrong.  Let's call it <perm>, as the value is used to set the
permission bits.

Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-init.txt

@@ -79,7 +79,7 @@ repository.  If not specified, fall back to the default name (currently
 `master`, but this is subject to change in the future; the name can be
 customized via the `init.defaultBranch` configuration variable).
 
---shared[=(false|true|umask|group|all|world|everybody|<umask>)]::
+--shared[=(false|true|umask|group|all|world|everybody|<perm>)]::
 
 Specify that the Git repository is to be shared amongst several users.  This
 allows users belonging to the same group to push into that
@@ -110,15 +110,16 @@ the repository permissions.
 
 Same as 'group', but make the repository readable by all users.
 
-'<umask>'::
+'<perm>'::
 
-'<umask>' is a 3-digit octal number prefixed with `0` and each file
-will have mode '<umask>'. '<umask>' will override users' umask(2)
+'<perm>' is a 3-digit octal number prefixed with `0` and each file
+will have mode '<perm>'. '<perm>' will override users' umask(2)
 value (and not only loosen permissions as 'group' and 'all'
 does). '0640' will create a repository which is group-readable, but
 not group-writable or accessible to others. '0660' will create a repo
 that is readable and writable to the current user and group, but
-inaccessible to others.
+inaccessible to others (directories and executable files get their
+`x` bit from the `r` bit for corresponding classes of users).
 --
 
 By default, the configuration flag `receive.denyNonFastForwards` is enabled