docs: pass Doc instead of Iroh in example

[azzamsa]

Description

Since iroh/examples/client.rs is the first example users see, it's likely they will use this file as a starting point for their app skeleton. I find it easier to keep Doc in a struct and pass it around rather than using iroh::client::Iroh.

I've also added a comment about the content_bytes() method, noting its ability to accept either a Doc or client::Iroh.

Related:

• iroh/iroh-cli/src/commands/doc.rs

  Line 689 in 17c654e

  let bytes = entry.content_bytes(doc).await.map_err(read_failed)?;

• iroh/iroh/tests/sync.rs

  Line 1024 in 17c654e

  let content = entry.content_bytes(doc).await;

Breaking Changes

No.

Notes & open questions

Change checklist

• Self-review.
• Documentation updates if relevant.
• Tests if relevant.
• All breaking changes documented.

[Frando]

LGTM!