Some code blocks missing syntax highlighting in docs

[anandijain]

In the docs, I noticed that there are multiple examples of jldoctest dataframe or julia missing from the beginning of code blocks.

It is easy enough to make a PR, but I'm wondering if there is a reason that certain ones have it and others don't.

Thanks!

[bkamins]

There is no reason currently.

At some point we want to turn on checking if the documentation actually reproduces the current behavior - then we should clean this thing up.

So in short - the PR would be really welcome, but then it would be preferable to make sure that examples actually pass tests when docstring checking is turned on.

[bkamins]

This is done now