hdf5doc

Description:
    Add notes that:
    -- H5Fopen, H5Pset_fclose_degree: 
         A multiply-opened file must use the same file close degree
         setting (H5Pset_fclose_degree) for each open instance.  
         If an H5Fopen call uses a file close degree setting different from
         that of an earlier open operation, the new H5Fopen call will fail.  
    -- H5Fopen, H5Lcreate_external, H5Pset_elink_fapl, H5Pset_elink_cb:
         When a file is opened by means of an external link, it is always
         opened with the weak file close degree setting, H5F_CLOSE_WEAK.
    Fixes HDFFV-8078.
    -- H5Pset_fclose_degree only:
         Update HTML and formatting. 
Tested:  Firefox