doc: add return type to Blocklist.isBlocklist

[motokaneyuki]

This adds missing return type for Blocklist.isBlocklist.

According to what this function returns:

node/lib/internal/blocklist.js

Lines 79 to 81 in 2d32b18

	static isBlockList(value) {
	return value?.[kHandle] !== undefined;
	}

Refs: nodejs/doc-kit#953

[nodejs-github-bot]

Review requested:

• @nodejs/net

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