documentation: Make JavaDoc of Grid#setClassNameGenerator more precise

[TatuLund]

Classname generator is cell based. If using Grid#setClassNameGenerator, it populates each cell in the row with the classname, not the row element. This is important to know, as it rules out certain styling options.

[TatuLund]

See also: https://vaadin.com/forum/thread/18531424/how-to-style-edited-row-in-grid-vaadin-14

[vaadin-bot]

SonarQube analysis reported 32 issues

• 2 critical
• 8 major
• 19 minor
• 3 info

Top 10 extra issues

Note: The following issues were found on lines that were not modified in the pull request. Because these issues can't be reported as line comments, they are summarized here:

1. Grid.java#L2397: Remove usage of generic wildcard type.
2. Grid.java#L3155: Define a constant instead of duplicating this literal "direction" 4 times.
3. Grid.java#: This file has 1,847 lines, which is greater than 750 authorized. Split it into smaller files.
4. Grid.java#L1933: Call "Optional#isPresent()" before accessing the value.
5. Grid.java#L1934: Either log or rethrow this exception.
6. Grid.java#L1953: Either log or rethrow this exception.
7. Grid.java#L2963: Call "Optional#isPresent()" before accessing the value.
8. Grid.java#L2973: Call "Optional#isPresent()" before accessing the value.
9. Grid.java#L3706: Rename "editor" which hides the field declared at line 1190.
10. Grid.java#L4117: Remove this unused method parameter "event".