Docs: `multivariate.md`

[Andy Nowacki]

The docs in docs/multivariate.md should be updated to:

1. Clarify the keyword arguments for multispec. They are currently hard to follow.
2. Clarify whether mt_ccvf or mt_acvf is the correct function. The mt_ccvf section first refers to the latter, and then the former, and seems to suggest they are the same.
3. Document all keywords

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[Charlotte Haley]

Updates to multivariate.md have been made here.

[Andy Nowacki]

Thanks. These address my comment, but I guess require the online docs before this can be closed.

[Charlotte Haley]

After implementing the Documenter.jl solution, I think the changes addressing the above are complete. Thanks.