[docs] add more info to allow_redefinition

[KotlinIsland]

I noticed the docs don't mention a rule of allow_redefinition

[hauntsaninja]

Thanks! IIRC the exact condition is "if the variable has not been used since it was assigned to", e.g. the following is allowed:

def process(items: list[str]) -> None:
    # 'items' has type list[str]
    items = [item.split() for item in items]
    # 'items' now has type list[list[str]]
    print(items)
    items = "mypy"

(I haven't read that part of the code in a while, so I might be mistaken)