[lib] Explain auto-created Farcaster mutual friend threads better

Summary:
Right now, the language we use to explain this is a little confusing. This diff clarifies what's going on, and avoids implying that one user created the chat.

This addresses ENG-9511.

Depends on D13687

Test Plan:

Reviewers: tomek

Reviewed By: tomek

Differential Revision: https://phab.comm.dev/D13688