Update Description for Goal Revenue Field to Include Reference to Custom Revenue Option

[bojana-z]

The current description for the goal revenue field in Matomo is unclear. It suggests that users can only set a specific revenue value when creating or editing a goal, but users can also manually enter a custom revenue value.

It would be helpful to add a reference or a link in the goal revenue field's description to the guide (https://developer.matomo.org/guides/tracking-javascript-guide#manually-trigger-goal-conversions) explaining that custom revenue can be manually added, as users may not be aware of this feature. This would provide clearer instructions for users when they are setting up goals.