Update memtrace analyzer docs and tools for skipped headers when fast-forwarding

The drmemtrace scheduler allows the user to specify regions of interest in the trace.

https://github.com/DynamoRIO/dynamorio/blob/f14c76c19800e91eaf7053837073d02a3fbfd377/clients/drcachesim/scheduler/scheduler.h#L271

As documented, when this is specified, entries outside these regions are simply skipped. What is not documented explicitly, is that this may include the trace header entries, such as TRACE_MARKER_TYPE_FILETYPE.

Some trace analysis tools rely on seeing the TRACE_MARKER_TYPE_FILETYPE marker and fail if they don't see it, particularly opcode_mix. E.g., https://github.com/DynamoRIO/dynamorio/blob/f14c76c19800e91eaf7053837073d02a3fbfd377/clients/drcachesim/tools/opcode_mix.cpp#L194.

This issue is to fix such tools and also improve documentation.