docs: GitLab comment strategy missing validation failure note

[chmouel]

The GitLab comment strategy documentation section is missing an important note about validation failure comments that is currently only documented in the GitHub section.

Problem:

• The comment strategy note only appears in the GitHub section
• Users may not understand that validation failure comments are always posted, even with enabled
• This creates inconsistency between GitHub and GitLab documentation

Solution:
Add the missing note to the GitLab section explaining that only applies to status comments and validation failures will always be posted.

Reference:
This issue was identified in internal Slack discussion about GitLab comment spam and the behavior of .