docs(utils): document why fieldCaches is keyed by EventType (#869)

fieldCaches had no comment, while the accessorCaches added alongside it carries a long one explaining why it is keyed by datasource instead. That left the more surprising of the two undocumented.

Records the invariant the EventType key depends on: a FieldAccessor decodes at a fixed offset in its own datasource's layout, so sharing a cache across an event type is only safe while one tracer is registered per event type, as RegisterTracer's map assignment enforces. Points readers at localFieldAccessor for reads that must not rely on it.

Comment-only; no behavioural change.

Docs-exempt: comment-only change, no behavioural or API change

Signed-off-by: Ben <ben@armosec.io>