Game Frontend: highlight the warning about persistent storage

[roberthbailey]

This is a minor thing, but I think you should better highlight this note:

Note: Open Match does not guarantee persistent storage and hence should not be used as the authoritative source for Game Client assignment information. Once the assignment for a Ticket is fetched, the Game Frontent should (if necessary) persist this information in its own persistent storage for future lookup.

@markmandel recently switched agones to using a really nice UI for highlighting notes, warnings, etc in googleforgames/agones#1152 but if your version of docsy doesn't support that then using the > syntax would still be preferable to just making it look like a normal paragraph.