Revise doc/state-contents.txt to be accurate

The doc/state-contents.txt file is out of date, and missing a bunch of documentation.

We should have accurate documentation for all of the state file's contents.

The following elements are undocumented:

• BWHistoryIPv6ReadEnds
• BWHistoryIPv6ReadInterval
• BWHistoryIPv6ReadValues
• BWHistoryIPv6ReadMaxima
• BWHistoryIPv6WriteEnds
• BWHistoryIPv6WriteInterval
• BWHistoryIPv6WriteValues
• BWHistoryIPv6WriteMaxima
• Guard
• TotalBuildTimes
• CircuitBuildAbandonedCount
• "CircuitBuildTimeBin"
• "BuildtimeHistogram"
• "MinutesSinceUserActivity"
• "Dormant"

The follow elements are obsolete:

• "EntryGuard"
• "EntryGuardDownSince"
• "EntryGuardUnlistedSince"
• "EntryGuardAddedBy"

These are obsolete and undocumented:

• "EntryGuardPathBias"
• "EntryGuardPathUseBias"
• HidServRevCounter