DOC import: fix nested comments

See 2.9.70 (FBKF) in the spec, the ibkl field should be used to find the
position in PlcfBkf.

Previously we assumed that PlcfBkl and PlcfBkf is parallel for
annotations, as but that's not true for nested comments.

Change-Id: I76a6a757b41d2f9b7fe7fe9c9e01a92c251f7b32