docs: Clarify that max duration is required

[Dan-Heath]

There is a discrepency between the UI and the docs over the Max Duration field in the UI. The UI says it is optional, but it appears it is required. This was documented incorrectly in the UI steps for creating an alias.

This PR corrects the incorrect step in the creating an alias procedure, and adds some additional text to the targets domain model and CLI topics to reinforce that the max duration is required and Boundary uses the default value if the user does not specify a value.

View the updates in the preview deployment:

• Aliases > Create an alias during target creation (step 4 on UI tab, updated Maximum Duration definition)
• Targets > Attributes (updated session_max_seconds definition)
• targets create CLI doc > Usages by type (updated session_max_seconds definition on SSH and TCP tabs)
• targets update CLI doc > Usages by type (updated session_max_seconds definition on SSH and TCP tabs)