Clarify wallet_invokeMethod params type in Multichain API docs

[kinderjoypresents]

This PR clarifies the params type in the wallet_invokeMethod request schema. JSON-RPC parameters may be either an object or an array, so the docs now reflect that.

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Note

Low Risk
Low risk documentation-only changes; the main impact is potential developer guidance changes around JSON-RPC params accepting arrays as well as objects.

Overview
Clarifies the wallet_invokeMethod request schema by documenting params as object | array to match JSON-RPC usage.

Also tidies multichain-api.md formatting (admonition blocks, JSON example array formatting, and ensures a trailing newline) without changing documented behavior.

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