Handling of arguments with 1e vs 2e differences

[jennybc]

While reviewing #1426, it occurs to me that the name_repair argument is completely unsupported in 1e. readr 1e has its own DIY name repair strategies hard-wired in. We should document that.

And now that I'm thinking about this, I'm quite sure other arguments are in a similar boat, i.e. only have an effect in 1e or have different behaviours in 1e vs 2e. We should have some standard wording for this.

[jennybc]

Thinking about a gradual process for phasing out 1e, we should also think about warning when a 1e user uses an argument that only exists for 2e. As it stands, we (almost?) always just quietly ignore that.

[jennybc]

Clearly related to tidyverse/vroom#461