Document more VSL tags
Document SLT_ExpKill

Use an attribute like syntax for these log records to make the
documentation more readable.
Document SLT_WorkThread log records
Add masked by default notes to appropriate log records.
Document a couple of VSL log records
Document the backend related VSL records.

Change what is logged in a couple of places for consistency.
Document the last of the VSL records
Format BereqEnd in the same way as the other tags
Emit SLT_HitPass log records
Flag unused VSL tags in the definition table.

Add a flags field to the VSL tag definition table, and use this to
flag unused records.
Use a tag flag to mark binary log records