docs: remove default iavl-cache-size invalid value

[mdumais-stratos]

Description

Closes: #11382

The memory leak from #11382 is caused by the iavl-cache-size being too large (500 000 by default). HistoricalInfo objects therefore accumulate in memory over time.

There is already a solution added by this PR to change the iavl-cache-size. It requires using the server.DefaultBaseappOptions, or directly calling baseapp.SetIAVLCacheSize when creating the app object, and changing the iavl-cache-size value in app.toml.

This PR updates the docs to make it clear that the iavl-cache-size parameter is a number of nodes, and not an amount of data.

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[julienrbrt]

ACK

[tac0turtle]

the default here https://github.com/cosmos/cosmos-sdk/blob/main/store/iavl/store.go#L25 is probably what we should also change to be smaller