Potentially incorrect comments in src/batch.rs

[mpguerra]

Motivation

We want to track all of the findings from the zebra audit, no matter how minor.

Details

Some of the comments in src/batch.rs seem slightly off:

• On line 146, the documentation of the verify() function includes a warning about outputs differing between batched and individual verifications; however, the whole point of ZIP 215 and the ed25519-zebra crate is indeed to ensure that batched and nonbatched verification always yield identical results on the same signatures.
• On line 154, the described verification equation does not include the multiplication by the cofactor. Such an equation would indeed lead to differences between batched and non-batched outputs. Fortunately, the implementation itself includes the multiplication by the cofactor (line 214).