x/tools/gopls: documentation not shown in completion results for fields of anonymous structs

[stamblerre]

Repro:

func TestReverse(t *testing.T) {
	for _, c := range []struct {
		// input to the test
		in   string
		want string
	}{
		{"Hello, world", "dlrow ,olleH"},
		{"Hello, 世界", "界世 ,olleH"},
		{"", ""},
	} {
		// <complete c.in here>
	}
}

There will be no documentation for c.in.