Document the default b-tree k values in the file format spec

[fortnern]

For version 1 b-trees, knowing the "k" value (related to the node size) is necessary to parse the file format. In some circumstances, this k value is not stored in the file and it is implied that it is the default value. However, the file format spec never lists what these default values are. They are:

For chunk indices: 32
For group symbol tables (internal nodes): 16
For group symbol tables (leaf node): 4

These should be listed in the file format spec because without knowing them it is impossible to parse or create an HDF5 file that uses the implied defaults.

The defaults are implied under one of these conditions:

• For a version 0 superblock the chunk index k value is never stored, it is always the default 32
• For a version 3+ superblock if the B-tree k values message is not present in the superblock they are assumed to be the default values