[DOC] Add note for cosine similarity vector space that vectors cant be 0 vec

[jmazanec15]

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Tell us about your request. Provide a summary of the request and all versions that are affected.
The cosine similarity metric for k-NN is computed as (a . b)/(|a|*|b|) where "." means dotproduct and || means the magnitude of the vector. If a or b is all zero's, this would lead to a divide by 0 exception. So, in opensearch-project/k-NN#1501, we are rejecting requests for indices that use cosine similarity if the vector is 0.

For this, we need to update the documentation to tell users that we will reject these requests. I think the best place to do this would be here:

1. https://opensearch.org/docs/latest/search-plugins/knn/approximate-knn/#spaces
2. https://opensearch.org/docs/latest/search-plugins/knn/approximate-knn/#spaces
3. https://opensearch.org/docs/latest/search-plugins/knn/knn-score-script/#spaces

What other resources are available? Provide links to related issues, POCs, steps for testing, etc.
Related to this PR: opensearch-project/k-NN#1501.

[hdhalter]

Thanks, @jmazanec15! Will this be part of the 2.13, or should we backport it to 2.12?

[jmazanec15]

@hdhalter this will be part of 2.13