changefeedccl: Add a large doc comment

I found drawing out this diagram useful when working on this system,
perhaps it'll be useful to others as well.

Release note: None

pkg/ccl/changefeedccl/BUILD.bazel

@@ -8,6 +8,7 @@ go_library(
         "changefeed_dist.go",
         "changefeed_processors.go",
         "changefeed_stmt.go",
+        "doc.go",
         "encoder.go",
         "errors.go",
         "metrics.go",

pkg/ccl/changefeedccl/doc.go

@@ -0,0 +1,91 @@
+// Copyright 2021 The Cockroach Authors.
+//
+// Licensed as a CockroachDB Enterprise file under the Cockroach Community
+// License (the "License"); you may not use this file except in compliance with
+// the License. You may obtain a copy of the License at
+//
+//     https://github.com/cockroachdb/cockroach/blob/master/licenses/CCL.txt
+
+/*
+Package changefeedccl is the internal implementation behind
+changefeeds.
+
+Changefeeds emit KV events on user-specified tables to user-specified
+sinks.
+
+Changefeeds are built on top of rangefeeds, which provide a stream of
+KV events for a given keyspan as well as periodic "resolved
+timestamps" for those spans. For more information on rangefeeds see
+
+    docs/RFCS/20170613_range_feeds_storage_primitive.md
+
+The changefeed machinery encodes and delivers both the KV events
+and resolved timestamps to the sinks. It further uses the resolved
+timestamps to periodically checkpoint a changefeed's progress such
+that it can be resumed in the case of a failure.
+
+To ensure that we can correctly encode every KV returned by the
+rangefeed, changefeeds also monitor for schema changes.
+
+"Enterprise" changefeeds are all changefeeds with a sink. These
+feeds emit KV events to external systems and are run via the job
+system.
+
+"Sinkless" or "Experimental" changefeeds are changefeeds without a
+sink which emit rows back to the original sql node that issues the
+CREATE CHANGEFEED request.
+
+The major components of this system are:
+
+changfeedAggregator: Reads events from a kvfeed, encodes and emits
+KV events to the sink and forwards resolved to the changeFrontier.
+
+changeFrontier: Keeps track of the high-watermark of resolved
+timestamps seen across the spans we are tracking. Periodically, it
+emits resolved timestamps to the sink and checkpoints the
+changefeed progress in the job system.
+
+kvfeed: Coordinates the consumption of the rangefeed with the
+schemafeed. It starts a set of goroutines that consume the
+rangefeed events and forwards events back to the
+changefeedAggregator once the schema for the event is known.
+
+schemafeed: Periodically polls the table descriptors
+table. Rangefeed events are held until it is sure it knows the
+schema for the relevant table at the event's timestamp.
+
+                 +-----------------+
+  +------+       |                 |       +-----+
+  | sink |<------+  changeFrontier +------>| job |
+  +------+       |                 |       +-----+
+                 +--------+--------+
+                          ^
+                          |
+                  +-------+--------+
+  +------+        |                |
+  | sink +<-------+  changefeedAgg |<------------+
+  +------+        |                |             |
+                  +--+-------------+         chanBuffer
+                     |                           |
+                     v                    +------+------+
+                   +--------------+       |             |
+                   |              +------>|  copyFromTo +--+
+                   |  kvfeed      |       |             |  |
+                   |              |       +------+------+  |
+                   +--------+---+-+              ^         |
+                            |   |             memBuffer    |
+                            |   |                |         |
+                            |   |          +-----+------+  |   +-----------+
+                            |   |          |            |  |   |           |
+                            |   +--------> |physical    +----->| rangefeed |
+                            |              |   feed     |  |   |           |
+                            |              +------------+  |   +-----------+
+                            |                              |
+                            |                              |
+                            |              +------------+  |
+                            +------------> | schemafeed |<-|
+                                           |  (polls)   |
+                                           +------------+
+
+*/
+package changefeedccl