Update documentation for heap ID and vlen data

[mattjala]

Addresses #6233

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Important

Update H5.format.4.0.dox to correct item size description and clarify Global Heap ID usage for variable-length data.

• Documentation Updates:
  • Corrects item size description from 'L' to 'O' in H5.format.4.0.dox.
  • Adds note on additional fields preceding Global Heap IDs in data object storage for variable-length and reference data.
  • Clarifies storage of variable-length data in global heap, specifying 4-byte sequence length and Global Heap ID representation.

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[brtnfld]

Changes check out.