docs: show child bindings access in logMethod hook

[la2278647-arch]

Summary

Fixes #897 — the logMethod hook has no obvious way to access the child
logger's bindings, making use cases like "prefix the message with the
child context" non-obvious.

Change

The hook already receives the logger instance as this, and
this.bindings() exposes the bindings set via logger.child(). The
logMethod section of docs/api.md did not mention either fact.

Adds two short paragraphs and a second example after the existing
argument-flipping example:

Inside the hook, `this` is the logger instance the method was called on.
For a child logger, `this.bindings()` provides the bindings set via
logger.child(), which allows adapting the message based on the child context:

followed by a minimal example that prefixes the message with the child's
featureName binding — matching the exact request in #897 ("prepend that
name to message").

Verification

Both snippets verified against the repo's own code (Node 24):

const child = logger.child({ featureName: 'MY FEATURE' })
child.info('special')
// msg becomes "MY FEATURE: special"

this.bindings() inside logMethod returns {"featureName":"MY FEATURE"} on a child logger and {} on the root logger. ✓
Docs-only change, no code touched.