[docs] [reducers] Add power-assign to ImmutableUpdatePatterns

[shinout]

Add power-assign to docs of ImmutableUpdatePatterns.

It' an immutable updater of POJO using MongoDB's operator, easier access to nested values.

const obj = {
  count: 11,
  arr: ['value1'],
  bar: { baz: 'abc' },
}
const newObj = assign(obj, {
  $inc: { count: 1 }, // $inc: operator to increment number
  $push: { arr: 'value2' }, // $push: operator to push elements to array
  $unset: { 'bar.baz': '' }, // $unset: operator to unset a value
})

assert(newObj.count === 12)
assert(newObj.bar.hasOwnProperty('baz') === false)
assert.deepEqual(obj.arr, ['value1', 'value2'])

[markerikson]

Hi. Appreciate the PR, but this isn't something I'm looking to specifically highlight on this docs page right now. I see that you submitted a PR to list it in my addons catalog, though, and I'll take that.