[Codegen] Descriptions of array items are ignored

[MarshalX]

Example lexicon:

"tags": {
    "type": "array",
    "items": {
        "type": "string"
        "type": "string",
        "maxLength": 25,
        "description": "Items in this array are applied with OR filters. To apply AND filter, put all tags in the same string and separate using && characters"
    }
}

Actual behavior:

tags: t.Optional[t.List[str]] = None  #: Tags.

Expected behavior:

tags: t.Optional[t.List[str]] = None  #: Tags. Items in this array are applied with OR filters. To apply AND filter, put all tags in the same string and separate using && characters

So the description of the array must be combined with the description of the items. Found in #503