Document how VOL connectors should format object tokens

[jhendersonHDF]

HDF5's expectations for how the bytes of object tokens should be formatted need to be documented. Specifically, what needs to be documented is:

• Whether unused bytes of an object token (when the size of a token is less than H5O_MAX_TOKEN_SIZE) can be left uninitialized or if they should be set to a specific value or any value
• Whether there is a 1 to 1 mapping between an object and a particular object token byte stream or if multiple different object token byte streams can point to the same object
• The significance of the token_size field, as returned by the H5VL_FILE_GET_CONT_INFO VOL operation