Update H5Pset_fill_time description in the documentation.

According to the documentation, H5D_FILL_TIME_IFSET is the default. But this is not true for parallel, where the fill value is always written unless it is disabled. Other factors can influence the behavior, such as chunking, parallel compression, etc...

The description should be updated to reflect the scenarios where the default behavior differs from H5D_FILL_TIME_IFSET.