Fix docs that uses += to add an element in a list even though painless does not accept it.

[jimczi]

This change fixes an example in the docs that use += to add an element in a list even though this syntax is not accepted by painless.

[nik9000]

This looks like an opportunity to get further on #18160! If you convert these to // CONSOLE instead of curl then gradle docs:check will actually make sure the snippets work. You might need // TESTSETUP or // TEST[continued] to make the generated tests make sense, but it is so worth it.

[jimczi]

@nik9000 I am changing some of the internals for the update API in another PR. I'll add the console stuff in this PR if you don't mind:
#20166

[nik9000]

I don't mind!

[jimczi]

@jdconrad out of curiosity we were wondering with @nik9000 if that is rejected because it would be slow ?

[jdconrad]

@jimferenczi It's not that it would be slower (for static types), but there are two downsides I see. The first one is added code complexity. For each overloaded operator there must be a section of specialized code to identify the list and call the appropriate method. We picked some of these that we saw as the most common/useful and decided to not have others. The second reason is added complexity for the user. Having more than one way of doing the same thing leads to the question of is one way better than the other. And at times one way is actually better than the other just because of the types the operator works on (specifically dynamic types) which can lead to even more confusion.

[jimczi]

Makes perfect sense, thanks @jdconrad