Fix container.atPosition() API doc

[ybiquitous]

This PR fixes the API doc for container.atPosition(line, column).

I referred to the following code for the description and example:

postcss-selector-parser/src/__tests__/container.js

Lines 353 to 375 in 54f71ef

	test('Container#atPosition first pseudo', (t) => {
	parse(':not(.foo),\n#foo > :matches(ol, ul)', (root) => {
	let node = root.atPosition(1, 1);
	t.deepEqual(node.type, "pseudo");
	t.deepEqual(node.toString(), ":not(.foo)");
	});
	});
	test('Container#atPosition class in pseudo', (t) => {
	parse(':not(.foo),\n#foo > :matches(ol, ul)', (root) => {
	let node = root.atPosition(1, 6);
	t.deepEqual(node.type, "class");
	t.deepEqual(node.toString(), ".foo");
	});
	});
	test('Container#atPosition id in second selector', (t) => {
	parse(':not(.foo),\n#foo > :matches(ol, ul)', (root) => {
	let node = root.atPosition(2, 1);
	t.deepEqual(node.type, "id");
	t.deepEqual(node.toString(), "\n#foo");
	});
	});

postcss-selector-parser/src/selectors/container.js

Lines 128 to 141 in 54f71ef

	/**
	* Return the most specific node at the line and column number given.
	* The source location is based on the original parsed location, locations aren't
	* updated as selector nodes are mutated.
	*
	* Note that this location is relative to the location of the first character
	* of the selector, and not the location of the selector in the overall document
	* when used in conjunction with postcss.
	*
	* If not found, returns undefined.
	* @param {number} line The line number of the node to find. (1-based index)
	* @param {number} col The column number of the node to find. (1-based index)
	*/
	atPosition (line, col) {