OvmfPkg/VirtioNetDxe: document Rx/Tx allocs and mappings on Init/Shutdown

OvmfPkg/VirtioNetDxe: document Rx/Tx allocs and mappings on Init/Shutdown

Document the following actions on the

  EfiSimpleNetworkStarted <-> EfiSimpleNetworkInitialized

state transitions:

* from commit 46b11f00ac70 ("OvmfPkg/VirtioNetDxe: alloc RxBuf using
  AllocateSharedPages()", 2017-09-14):

  VirtioNetInitRx ->
  { VirtIo->AllocateSharedPages, VirtioMapAllBytesInSharedBuffer }

  VirtioNetShutdownRx ->
  { VirtIo->UnmapSharedBuffer, VirtIo->FreeSharedPages }

* from commit 891f016c1b99 ("OvmfPkg/VirtioNetDxe: dynamically alloc
  transmit header", 2017-09-14):

  VirtioNetInitTx ->
  { VirtIo->AllocateSharedPages, VirtioMapAllBytesInSharedBuffer }

  VirtioNetShutdownTx ->
  { VirtIo->UnmapSharedBuffer, VirtIo->FreeSharedPages }

Cc: Brijesh Singh <brijesh.singh@amd.com>
Cc: Jordan Justen <jordan.l.justen@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Brijesh Singh <brijesh.singh@amd.com>