Document `read_view.with` method

The new read view method :with() executes a function with a guarantee
that the read view object will remain operational until the function
returns, even if it's closed from another fiber.

Usage:

local rv = box.read_view.open()

-- ...

rv:with(function()
    -- do something with the read view
end)

If the read view object is closed (that is :close() is called) while
a function passed to :with() is still running, the read view status
will change to close_pending and remain such until the function
returns, after which it will change to closed and the read view
resources will be freed. A read view in the close_pending state can
be accessed just like an open read view but calling :with() on it will
raise an error.

A function called using :with() is passed the read view object as the
first argument, which may be used instead of capturing it in an upvalue,
and the rest of the arguments passed to :with() after the function.
The :with() method returns the same value as the called function.
If the called function raises an error, the error is re-raised.

For example:

local function read_view_avg(rv, space_name, field_name)
    local count = 0
    local sum = 0
    for _, tuple in rv.space[space_name]:pairs() do
        sum = sum + tuple[field_name]
        count = count + 1
        if count % 1000 == 0 then
            -- yield once in a while so as not to block other fibers
            fiber.yield()
        end
    end
    return sum / count
end

local rv = box.read_view.open()
local avg = rv:with(read_view_avg, 'my_space', 'some_field')

Requested by @locker in https://github.com/tarantool/tarantool/commit/2952a9afcaed1b7ae33267cdd703e98c365bcf3a.

Search the Tarantool documentation for the existing box.read_view.open and read view API reference, then place the new :with() method alongside it. Document its arguments, callback behavior, return values, error propagation, and close_pending semantics using the issue examples; done means the method is discoverable and its documented behavior matches the request.