Add a comment on the recommended `bool Entry(T& mutable_entry, size_t, size_t) override { .. }` syntax.

[dkorolev]

[dkorolev]

CC @sompylasar.

[sompylasar]

Refs: https://github.com/KnowSheet/Sherlock/blob/master/sherlock.h#L59

[sompylasar]

Refs: KnowSheet/AgreeDisagreeDemo#12 (comment)