span, doc: provide span.h context and explain lifetimebound definition

src/span.h

@@ -30,7 +30,11 @@
 
 /** 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:
  *