[Tokenizer doc] Clarification about `add_prefix_space`
# What does this PR do?
Adresses #17391,  updates the documentation that suggested to use `add_prefix_space` when calling the tokenizer
_The documentation is not available anymore as the PR was closed or merged._<|||||>This blog post on constrained decoding also uses `add_prefix_space` in `__call__` https://huggingface.co/blog/constrained-beam-search
<|||||>The blog is not hosted on `transformers` but on `blog`, will open a PR for that too later on, thanks for the catch 😉 