i40e: add description and modify interrupts configuration procedure

Add description for values written into registers QINT_XXXX
and small cosmetic changes for MSI/LEGACY interrupts
configuration in the same way as for MSI-X.
Descriptions confirm the code is written correctly and
make the code clear. Small cosmetic changes for MSI/LEGACY
interrupts make code clear in the same manner as for MSI-X
interrupts.

Signed-off-by: Jaroslaw Gawin <jaroslawx.gawin@intel.com>
Signed-off-by: Andrii Staikov <andrii.staikov@intel.com>
Tested-by: Gurucharan <gurucharanx.g@intel.com> (A Contingent worker at Intel)
Signed-off-by: Tony Nguyen <anthony.l.nguyen@intel.com>