[v1-rest bulk/observations] make it clear in docs that more than 2 entities are possible

[Carolyn Au]

https://juliawu.github.io/datacommons-docsite/api/rest/v1/bulk/observations/series

At the moment, judging from the examples and request format at the top of the page, it looks as if exactly 2 entities & variables are required. Can we make it clearer that it's 1 or more? Perhaps with ellipses, and an example that uses a different number of arguments.