docs: fill Client, WorkflowHandle, and ScheduleHandle method docstrings

Addresses zorporation/durable-workflow#387 and #388. Every public method
on Client (32 including get_cluster_info), WorkflowHandle (7), and
ScheduleHandle (7) now has prose that describes the semantic behavior,
not just the signature. Handle methods are intentionally terse and link
back to the corresponding Client method as the canonical source of truth.

The signal_workflow / query_workflow / update_workflow docstrings
describe the sender-side behavior; in-workflow handler decorators are
tracked separately in #390.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: durable-workflow-ops