Add an intro for some abstract ops sections

index.bs

@@ -628,6 +628,9 @@ href="https://github.com/whatwg/streams/issues/97">#97</a> for more information.
 
 <h3 id="rs-abstract-ops">General Readable Stream Abstract Operations</h3>
 
+The following abstract operations, unlike most in this specification, are meant to be generally useful by other
+specifications, instead of just being part of the implementation of this spec's classes.
+
 <h4 id="acquire-readable-stream-byob-reader" aoid="AcquireReadableStreamBYOBReader" throws>AcquireReadableStreamBYOBReader ( stream )</h4>
 
 This abstract operation is meant to be called from other specifications that may wish to acquire a <a>BYOB reader</a>
@@ -799,6 +802,17 @@ called with argument <var>reason</var>, it performs the following steps:
 
 <h3 id="rs-abstract-ops">Readable Stream Abstract Operations Used by Controllers</h3>
 
+In terms of specification factoring, the way that the <code>ReadableStream</code> class encapsulates the behavior of
+both simple readable streams and <a>readable byte streams</a> into a single class is by centralizing most of the
+potentially-varying logic inside the two controller classes, <code>ReadableStreamDefaultController</code> and
+<code>ReadableStreamBYOBController</code>. Those classes define most of the stateful internal slots and abstract
+operations for how a stream's <a>internal queue</a> is managed and how it interfaces with its <a>underlying source</a>
+or <a>underlying byte source</a>.
+
+The abstract operations in this section are interfaces that are used by the controller implementations to affect their
+associated <code>ReadableStream</code> object, translating those internal state changes into developer-facing results
+visible through the <code>ReadableStream</code>'s public API.
+
 <h4 id="readable-stream-add-read-request" aoid="ReadableStreamAddReadRequest" nothrow>ReadableStreamAddReadRequest ( stream )</h4>
 
 <emu-alg>