[e.cash] Update FAQ question about claiming split BCH

Summary: The old answer wasn't very informative. Better to direct people to the blog post.

Test Plan: Ran it locally, read answer and make sure it makes sense.

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D14345