Clarified early reading of the request body

[xeioex]

In continuation from discussion started in #321.

The client_body_early_read directive changes when and how the request body becomes available, but that is currently documented only on the directive itself. Related places still describe the pre-1.31.5 state or leave the interaction unstated:

• $request_body claims the value is available only in locations processed by proxy_pass/fastcgi_pass/uwsgi_pass/scgi_pass; with early reading it is available in any
  location.
• location: a predicate whose variable depends on the request body never matches unless the body is read early, since predicates are evaluated before the body is read. The failure is silent, the request falls back to the prefix location.
• client_body_in_file_only: the fact that it is ignored is stated only in client_body_early_read, not in the directive itself.
• client_body_early_read: it is not said that the body is read before a location is chosen, which is what makes the server-block settings, the ignored client_body_in_file_only and the availability in any location follow. The client_max_body_size/client_body_buffer_size pair in the example is unexplained: with a body larger than the buffer the body is written to a temporary file and $request_body is not set.