Update pathNode documentation to reflect reality

Neither fidRefs or children are (directly) synchronized by mu. Remove
the preconditions that say so.

That said, the surrounding does enforce some synchronization guarantees
(e.g., fidRef.renameChildTo does not atomically replace the child in the
maps). I've tried to note the need for callers to do this
synchronization.

I've also renamed the maps to what are (IMO) clearer names. As is, it is
not obvious that pathNode.fidRefs is a map of *child* fidRefs rather
than self fidRefs.

PiperOrigin-RevId: 254446965

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