Positional vs keyward arg doc improvements

[robhudson]

When working on projects that use the Python elasticsearch client I find myself jumping to the code to see which arguments are required (positional) and which are optional with reasonable default values.

I think it would benefit the docs to make this clearer.

[honzakral]

The docs mention that you should always use keyword arguments - mostly for readibility, but also because of forwards compatibility - if an argument goes from required to optional or vice versa, relying on the order would cause errors.

If you know how to make this more prominent in the docs I'd be grateful.