feat: update ambiguous wording about RPC block parameter

[Thegaram]

Description

The current JSON-RPC page has a section about the "extra default block parameter". The words "extra" and "default" are ambiguous and confusing because the spec actually requires this parameter.

Related Issue

N/A

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[github-actions[bot]]

This issue is stale because it has been open 30 days with no activity.

[Thegaram]

@minimalsm Please kindly triage and decide to merge or close.

[Thegaram]

@wackerow Please kindly triage and decide to merge or close.

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