message: span, doc: provide span.h context and explain lifetimebound definition
diff: /** A Span is an object that can refer to a contiguous sequence of objects.
  *
  - * It implements a subset of C++20's std::span.
  + * This file implements a subset of C++20's std::span. It can be considered
  + * temporary compatibility code until C++20 and is designed to be a
  + * self-contained abstraction without depending on other project files. For this
  + * reason, Clang lifetimebound is defined here instead of including
  + * <attributes.h>, which also defines it.
  *
  * Things to be aware of when writing code that deals with Spans:
  *
  