Identifiers should be nouns

When we refer to an identifier in a comment followed by a linking verb, Harper throws a false-positive because it does not mark the identifier as being a noun.

The `is` is marked as improper here:

```java
/** The user the JWT is associated with. */
String usr;
```

Start by reproducing the JavaDoc example in the issue and tracing how Harper classifies identifiers in comments before a linking verb. Done means the `is` in that example is no longer reported as improper, with existing grammar checks still behaving as expected.

Written by the indexing model from the issue text.