Improve help texts

[waldyrious]

• Rename "Fade out speed" to "Particle trace persistence". The old name was vaguer and could be interpreted to mean that a higher number leads the particles to fade out quicker, while in fact it's the opposite.
• Format default color option in the same way as the other defaults/recommended values
• Add punctuation to all sentences that were missing it.

Note: I'm open to discuss other possible names for the trace fade out setting, if the one I'm proposing isn't to your liking.