KEYS: Update the keyrings documentation for match changes

				const char *callout_info);

    This is used to request a key or keyring with a description that matches

    the description specified according to the key type's match function. This

    permits approximate matching to occur. If callout_string is not NULL, then

    /sbin/request-key will be invoked in an attempt to obtain the key from

    userspace. In that case, callout_string will be passed as an argument to

    the program.

    the description specified according to the key type's match_preparse()

    method. This permits approximate matching to occur. If callout_string is

    not NULL, then /sbin/request-key will be invoked in an attempt to obtain

    the key from userspace. In that case, callout_string will be passed as an

    argument to the program.

    Should the function fail error ENOKEY, EKEYEXPIRED or EKEYREVOKED will be

    returned.

     It is safe to sleep in this method.

 (*) int (*match)(const struct key *key, const void *desc);

 (*) int (*match_preparse)(struct key_match_data *match_data);

     This method is called to match a key against a description. It should

     return non-zero if the two match, zero if they don't.

     This method is optional.  It is called when a key search is about to be

     performed.  It is given the following structure:

     This method should not need to lock the key in any way. The type and

     description can be considered invariant, and the payload should not be

     accessed (the key may not yet be instantiated).

	struct key_match_data {

		bool (*cmp)(const struct key *key,

			    const struct key_match_data *match_data);

		const void	*raw_data;

		void		*preparsed;

		unsigned	lookup_type;

	};

     It is not safe to sleep in this method; the caller may hold spinlocks.

     On entry, raw_data will be pointing to the criteria to be used in matching

     a key by the caller and should not be modified.  (*cmp)() will be pointing

     to the default matcher function (which does an exact description match

     against raw_data) and lookup_type will be set to indicate a direct lookup.

     The following lookup_type values are available:

      [*] KEYRING_SEARCH_LOOKUP_DIRECT - A direct lookup hashes the type and

      	  description to narrow down the search to a small number of keys.

      [*] KEYRING_SEARCH_LOOKUP_ITERATE - An iterative lookup walks all the

      	  keys in the keyring until one is matched.  This must be used for any

      	  search that's not doing a simple direct match on the key description.

     The method may set cmp to point to a function of its choice that does some

     other form of match, may set lookup_type to KEYRING_SEARCH_LOOKUP_ITERATE

     and may attach something to the preparsed pointer for use by (*cmp)().

     (*cmp)() should return true if a key matches and false otherwise.

     If preparsed is set, it may be necessary to use the match_free() method to

     clean it up.

     The method should return 0 if successful or a negative error code

     otherwise.

     It is permitted to sleep in this method, but (*cmp)() may not sleep as

     locks will be held over it.

     If match_preparse() is not provided, keys of this type will be matched

     exactly by their description.

 (*) void (*match_free)(struct key_match_data *match_data);

     This method is optional.  If given, it called to clean up

     match_data->preparsed after a successful call to match_preparse().

 (*) void (*revoke)(struct key *key);