webpmux man page: Clarify some title, descriptions and examples

Based on the feedback here:
https://groups.google.com/a/webmproject.org/d/msg/webp-discuss/qY6rWJLqRTY/pF8oSj8DOGYJ

Change-Id: I9119ea8e211ffb269026010e3c590385bc6a9f17