LineLayer flowline 单箭头文档缺失

[zyl-fz]

翻看issues列表时发现可通过flowline绘制单箭头，地址为：https://github.com/antvis/L7/issues/2159， 但目前官网缺少该文档，尤其style中的symbol完全无任何信息可检索

[lvisei]

TODO: Docs #2159

L7/packages/layers/src/core/interface.ts

Lines 329 to 353 in e61188a

	export type ArrowType =
	| 'circle'
	| 'triangle'
	| 'rect'
	| 'diamond'
	| 'classic'
	| 'halfTriangle'
	| 'none';
	export interface IArrowOptions {
	type: ArrowType;
	width?: number;
	height?: number;
	radius?: number;
	}
	export interface ILineSymbol {
	source: ArrowType | IArrowOptions;
	target: ArrowType | IArrowOptions;
	}
	export interface IFlowLineStyleOptions extends IBaseLayerStyleOptions {
	gapWidth?: number;
	offsets?: [number, number];
	stroke?: string;
	strokeOpacity?: number;
	strokeWidth?: number;
	symbol?: ILineSymbol;