Changed the time for DistPacketGenerator to send the first packet.

[cying17]

This PR changed to let DistPacketGenerator send the first packet right after initial_delay.

Description

Changed to let DistPacketGenerator send the first packet right after initial_delay.
Improved some comments.

How has this been tested?

python examples/basic.py

Types of changes

• Bug fix (non-breaking change which fixes an issue) Fixes #
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.