Document how to disable upgrade notification

[jennymahmoudi]

When we have implemented the config to disable upgrade notifications in flox/flox#2534, we will need a place in docs where users can easily find "how to disable upgrade notifications" that includes the exact message wording.

The exact message wording will be:

ℹ️  Upgrades are available for packages in 'environmentname'. 
Use 'flox upgrade' to get the latest. 

And when we have implemented flox upgrade --dry-run in flox/flox#2356, the message will be:

ℹ️  Upgrades are available for packages in 'environmentname'. 
Use 'flox upgrade --dry-run' for details.