Documentation: Exporting: update description of d_splice_alias

   1/ provide the filehandle fragment routines described below.

   2/ make sure that d_splice_alias is used rather than d_add

      when ->lookup finds an inode for a given parent and name.

      Typically the ->lookup routine will end with a:

      If inode is NULL, d_splice_alias(inode, dentry) is eqivalent to

		d_add(dentry, inode), NULL

      Similarly, d_splice_alias(ERR_PTR(err), dentry) = ERR_PTR(err)

      Typically the ->lookup routine will simply end with a:

		return d_splice_alias(inode, dentry);

	}