`caf::flow`: backpressure, lifecycle and documentation defects

[shariarriday]

Bugs

• Make interval::request(0) a no-op — it arms the timer without demand and
  fire() underflows demand_ to SIZE_MAX, so the operator emits forever
  (op/interval.cpp).
• Honour from_external in stream_bridge_sub::do_dispose — deliver
  on_error(sec::disposed), else release_later(); today the observer gets no
  terminal event and is never released (detail/stream_bridge.cpp:142).
• Specify the request(0) contract on subscription::request and apply it
  uniformly — it aborts ucast/mcast under runtime checks, underflows
  interval, and is forwarded blindly by zip_with. Specify demand overflow
  behavior at the same time.
• Make zip_with credit each input only up to demand_ - buf.size()
  instead of forwarding the full demand to all inputs (op/zip_with.hpp:89).
• Document that publish gates upstream credit on its slowest subscriber,
  and consider an opt-in overflow policy for slow subscribers
  (op/publish.hpp:143).
• Document and test buffer's deferral of terminal events while a partial
  batch is pending (op/buffer.hpp).

Documentation — manual/core/DataFlows.rst

• Specify the backpressure contract: who calls request, whether
  request(0) is legal, whether demand saturates, how lossy operators spend
  demand, and how cancel differs from an external dispose.
• Replace dispose with cancel in the subscription description.
• Correct the merge concurrency default — eight applies only to the
  zero-argument form; the variadic form uses sizeof...(Inputs) + 1.
• Fix the "Prefix and Tail" section to describe prefix_and_tail, and note
  that it emits nothing when the source ends before n items.
• Replace the stale names callable_source and limit_step with
  gen::from_callable and step::take.
• Classify the APIs as hot, cold, connectable, single-subscribe or
  replaying (multicaster, from_resource, cache, publish, ref_count).
• Document the surprising terminal semantics of element_at, to_vector,
  on_error_return, on_error_return_item and reduce.
• Cover the missing APIs: the source factories, zip_with,
  combine_latest, the multicasting and connectable APIs, cache, single,
  retry, on_error_resume_next, the do_on_* hooks, the bridging and actor
  integration APIs, and the steps in flow/byte.hpp and flow/string.hpp.

Documentation — Doxygen

• Fix from_container and from_callable, which claim to emit ascending
  values (observable_builder.hpp).
• Fix the prediate typo in filter and take_while
  (observable_decl.hpp).
• Fix generation and transformation, described as blueprints for an
  observer instead of an observable (fwd.hpp).
• Replace the copy-pasted op::never class comments on op::retry and
  op::on_backpressure_buffer.

Tests

• prefix_and_tail: what a tail subscriber sees when the prefix
  subscription is disposed externally.
• Demand overflow under repeated large request(n).