Improve documentation for hvl_t structure

[bmribler]

When searching for the hvl_t structure on the HDF5 documentation site, it comes up like this:

So there are two hvl_t search results ( eg. https://docs.hdfgroup.org/hdf5/v1_14/structhvl__t.html ) and only by clicking on them one can find out that one entry is for FORTRAN, the other one is for the C API. This calls for improvement.

Also, I would recommend to mention the hvl_t data structure in the documentation of function H5Tvlen_create() such that it is clear that datasets need to be of this type. While I know that myself, this knowledge is somewhat mysterious from that function documentation.

Moreover, the function documentation mentions "Creating variable-length string datatypes", but that should come with a clickable link that directly references that topic:

https://docs.hdfgroup.org/hdf5/v1_14/group___v_l_e_n.html

(Extracted from issue HELP-2512)