docs: complete Google-style docstrings for the public API

[PabloPardoGarcia]

A docstring audit for the documentation site found most Observation and Generation methods undocumented and several public entry points at one-line stubs. The docs site generates its API reference from these docstrings, so they are now the single source of truth for reference documentation.

What changed (docstrings only, no behavior):

• Observation: class + all five methods (set_input, set_output, set_attribute, update, end), documenting serialization and the manual-vs-context-manager end() contract.
• Generation: class + all seven methods, documenting message normalization, default roles, the tokens-not-cost convention, and the emitted gen_ai.* attributes.
• observe(): full Args/Returns, incl. generator semantics and exception behavior.
• resolve_config(): precedence rules, env var names, clamping; GlassflowConfig class docstring.
• GlassflowClient + get_tracer, build_span_exporter, start_generation/start_as_current_generation Args/Returns, SpanKind variant descriptions.

Gates: ruff check, ruff format --check, mypy strict, pytest all green.