[FEATURE] Document completing a function from the viewSubmissionHandler

[zimeg]

Description of the problem being solved

Marking a function as complete from a viewSubmissionHandler requires using the client provided as an argument and making a call to functions.completeSuccess. A sample example is here but having this documented would be nice!

Requirements

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[zimeg]

Assigning this to myself and plan to tackle it soon, but anyone can feel free to pick it up! I'll toss 👀 on it once I open my editor.

[zimeg]

Maybe this points to https://api.slack.com/automation/interactive-modals#close-modal