feat: Document PGVECTOR_INITIALIZE_MAX_VECTOR_LENGTH

[alexandrescieux]

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Problem Description

I encountered the same issue as discussed in this discussion.

It seems that the following pull request has fixed the problem.

I can see the variable present in this file. However, I cannot find the corresponding configuration in the documentation.

Desired Solution you'd like

Document the existence of PGVECTOR_INITIALIZE_MAX_VECTOR_LENGTH and how to use it.

Alternatives Considered

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Additional Context

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