docs: correct public sketch semantics

[tisonkun]

Summary

• correct Bloom membership examples: positive results mean possibly present, while negative results mean definitely absent in normal operation
• document Hll4, Hll6, and Hll8 as isomorphic register representations with identical accuracy for the same lg_k
• explain how HLL union coupon-mode replay and array-mode downsampling determine the result's effective lg_k and error distribution
• correct the documented widths of the four 64-bit SketchSlice readers
• clarify that core Frequencies operations require Eq + Hash, while FrequentItemValue is required only for built-in serialization
• align Theta and Tuple Jaccard seed-error documentation with the empty-input fast paths

The broader post-invert Bloom contract remains tracked in #194.

Validation

• cargo x check
• cargo x test
• cargo x lint