docs/fingerprint: Add details on types of keys
docs/fingerprint: Add details on types of keys

BRANCH=none
BUG=none
TEST=none

Change-Id: I2c0b404ddfbd8e35fcc7455d93c1830167baa564
Signed-off-by: Tom Hughes <fc5c12f30a5ed2baa0581f44ee14ad1eb7b20d83@chromium.org>
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/ec/+/1922229
Reviewed-by: Craig Hesling <383db090fd8ef907a606ac8a9eccd2962bc489cf@chromium.org>
Commit-Queue: Craig Hesling <383db090fd8ef907a606ac8a9eccd2962bc489cf@chromium.org>

coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec