Wrap comments at 90 characters

#3 specifies that _code_ should be wrapped at 120 characters, but prose in the form of comments is hard to read at that width. 90 characters is an ideal wrap margin for Javadoc and other comments, as it strikes a good balance between readability and accommodating longer literals like fully-qualified classnames, URLs, and so forth without requiring wrapping. Also, the ratio between 120-character code lines and 90-character Javadoc lines is aesthetically pleasing on its own.

Unfortunately, IDEA does not (yet) support enforcing multiple wrap margins, e.g. one for code lines and one for comment lines like Eclipse has done for years. But recent IDEA versions _do_ support multiple visual guides, e.g. one at 120 for code and one at 90 for Javadoc. Our common IDEA code styles configuration has been updated to introduce the 90-character visual guide as a reminder about this.

See:
- https://youtrack.jetbrains.com/issue/IDEA-114661
- https://youtrack.jetbrains.com/issue/IDEA-114661#comment=27-794370

Start with the repository's common IDEA code styles configuration, which the issue identifies as the place for the additional visual guide. Check that comments and Javadoc are guided at 90 characters while code remains guided at 120, then verify the configuration reflects both margins.

Written by the indexing model from the issue text.