doc: add doc for Optional type #480

	name: test
	on:
	push:
	branches:
	- main
	# - v2
	pull_request:
	branches:
	- main
	# - v2
	jobs:
	build:
	runs-on: ubuntu-latest
	steps:
	- uses: actions/checkout@v2
	with:
	fetch-depth: 2
	- uses: actions/setup-go@v2
	with:
	go-version: "1.18"
	- name: Run coverage
	run: go test -v ./... -coverprofile=coverage.txt -covermode=atomic
	- name: Run govet
	run: go vet -v ./...
	- name: Upload coverage to Codecov
	run: bash <(curl -s https://codecov.io/bash)

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