Update file gitlab_agent_configuration.md

What does this MR do and why?

When Docs: Add details about default and max workspa... (!140440 - merged) was refactored, we missed one important point -

Changing this value immediately restarts the non-terminated workspaces of the agent making the new values take effect.

This needs to be called out explicitly and is part of the acceptance criteria for the parent issue - Docs: Add section for default_resources_per_wor... (#427145 - closed)