Document HDF5 dataset fill value record

This record is available since 7a0d017 (Added fillValue parameter to
the HDF5 file writer., 2015-09-22), but it was never properly
documented.

Document it under the metadata section as it is basically metadata
regarding the default value the readers should read when no value is
directly defined. This has been documented based on the corresponding
explanation from the HDF5 library function [1] that uses this value,
that is, H5Pset_fill_value().

[1] https://support.hdfgroup.org/documentation/hdf5/latest/group___d_c_p_l.html

Author: henriquesimoes

docs/ADCore/NDFileHDF5.rst

@@ -662,6 +662,12 @@ Parameters and Records
     - HDF5_dimAttDatasets
     - $(P)$(R)DimAttDatasets, $(P)$(R)DimAttDatasets_RBV
     - bo, bi
+  * - asynFloat64
+    - r/w
+    - Fill value for the dataset to be set in its creation property list.
+    - HDF5_fillValue
+    - $(P)$(R)FillValue, $(P)$(R)FillValue_RBV
+    - ao, ai
   * -
     -
     - **SWMR**