storelivenss: update storeliveness/doc.go with heartbeat smearing info

[dodeca12]

storeliveness/doc.go is outdated with respect to heartbeat smearing changes.

In particular, the Transport section doesn't have information about heartbeat smearing. Additionally, the Configuration section didn't detail the heartbeat smearing cluster setting
kv.store_liveness.heartbeat_smearing.enabled.

Added a dedicated bullet point in the Transport section (5.2) that explicitly describes heartbeat smearing as a feature to avoid goroutine spikes. Also updated the Configuration section (5.3) to detail that kv.store_liveness.heartbeat_smearing.enabled is available, and describes the behaviour of heartbeat sends when the cluster setting is enabled or disabled.

Informs: #156830
Release note: None

[blathers-crl]

It looks like your PR touches production code but doesn't add or edit any test code. Did you consider adding tests to your PR?

_{🦉 Hoot! I am a Blathers, a bot for CockroachDB. My owner is dev-inf.}

[cockroach-teamcity]

This change is 

[miraradeva]

Two small comments.

@miraradeva reviewed 1 of 1 files at r1, all commit messages.
Reviewable status: complete! 1 of 0 LGTMs obtained (waiting on @dodeca12)

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pkg/kv/kvserver/storeliveness/doc.go line 223 at r1 (raw file):

// `kv.store_liveness.heartbeat_smearing.enabled` cluster setting (enabled by
// default). When enabled, heartbeat sends are distributed over a certain
// duration heartbeat sends are distributed over a certain duration

nit: repeated words.

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pkg/kv/kvserver/storeliveness/doc.go line 227 at r1 (raw file):

// setting), with messages being sent at a certain interval (configured via the
// `kv.store_liveness.heartbeat_smearing.smear` cluster setting) to avoid
// spiking the number of runnable goroutines. When disabled, heartbeat sends are

Can you mention what the goroutines are? The per-node send queue processors, and the per-node gRPC connections.