[Docs] Clarify Whitespace tokenizer behaviour with tokens longer than 255 characters

[cbuescher]

Currently the Whitespace tokenizer splits tokens longer than 255 characters into separate tokens by default. This is suprising to some users (see #26601 for an example why this can be confusing). We should document this better (other tokenizers like the Standard tokenizer have some explanation for this in the docs where we talk about the max_token_length parameter). Maybe we should also check that other tokenizers exhibiting this behaviour have a small not in the docs.

[cbuescher]

Very long tokens like in #26601 might be an edge case that is rare and not necessary to support, but having a shorter max_token_length than the default 255 might be useful sometimes.

[cbuescher]

Documentation added on 5.6 and 6.0 branch, added support for overriding max_token_length ob 6.x and 7.0.