[DOC] - Messaging Character Limits Information Update

[kathryncc]

https://developer.signalwire.com/messaging/guides/general/messaging-character-limits

This sentence is out of date, under section "What is a Message Segment?":
Your project will be charged for each segment sent or received except for inbound SMS (not MMS) which are free of cost.

We currently charge for inbound SMS:
https://signalwire.com/pricing/messaging

That changed for new customers back in June 2023. It might be a good idea to have the rest of the info on this doc audited and verified.