Update info text of Function node

[HiroyasuNishiyama]

• Bugfix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)

Proposed changes

Setup code has been changed so that node.send can be used.
Since it is executed during the initialization process, the following nodes may not be able to receive the message.
This PR adds a note of caution about this.
Similarly, description about return value of setup process was added.

Checklist

• I have read the contribution guidelines
• For non-bugfix PRs, I have discussed this change on the forum/slack team.
• I have run grunt to verify the unit tests pass
• I have added suitable unit tests to cover the new/changed functionality