Document PVI1 bank format

[OPNA2608]

Current documentation only refers to PVI2:

libopenpmd/docs/pvi.txt

Lines 16 to 17 in ef45d61

	0x0000 Identifier (4b)
	"PVI2"

See BambooTracker/BambooTracker#308.

The bank file has the magic number PVI1, the PVI file parser checks for PVI2. From a cursory look the header looks fairly similar to what I've seen in PVI2 files.