docs: Document heightByRows performance implications (#919)

[alvarezguille]

Co-authored-by: Guille alvarezguille@users.noreply.github.com

[vaadin-bot]

SonarQube analysis reported 74 issues

• 4 critical
• 23 major
• 46 minor
• 1 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. GridDemo.java#L2249: Make "draggedItems" transient or serializable.
2. GridDemo.java#L2250: Make "draggedItem" transient or serializable.
3. Grid.java#L2320: Remove usage of generic wildcard type.
4. Grid.java#L2964: Define a constant instead of duplicating this literal "direction" 4 times.
5. GridDemo.java#: This file has 1,940 lines, which is greater than 750 authorized. Split it into smaller files.
6. GridDemo.java#L127: Constructor has 8 parameters, which is greater than 7 authorized.
7. GridDemo.java#L252: Define and throw a dedicated exception instead of using a generic one.
8. GridDemo.java#L805: Remove this useless assignment to local variable "ageColumn".
9. GridDemo.java#L1371: Extract the assignment out of this expression.
10. GridDemo.java#L1372: Extract the assignment out of this expression.