ARROW-16773: [Docs][Format] Document Run-Length encoding in Arrow columnar format

docs/source/format/Columnar.rst

@@ -765,6 +765,65 @@ application.
 We discuss dictionary encoding as it relates to serialization further
 below.
 
+.. _run-length-encoded-layout:
+
+Run-Length-encoded Layout
+-------------------------
+
+Run-Length is a data representation that represents data as sequences of the
+same value, called runs. Each run is represented as a value, and an integer
+describing how often this value is repeated.
+
+Any array can be run-length-encoded. A run-length encoded array has a single
+buffer holding as many signed 32-bit integers, as there are runs. The actual
+values are held in a child array, which is just a regular array.
+
+The values in the parent array buffer represent the length of each run. They do
+not hold the length of the respective run directly, but the accumulated length
+of all runs from the first to the current one. This allows relatively efficient
+random access from a logical index using binary search. The length of an
+individual run can be determined by subtracting two adjacent values.
+
+A run has to have a length of at least 1. This means the values in the
+accumulated run lengths buffer are all positive and in strictly ascending
+order.
+
+An accumulated run length cannot be null, therefore the parent array has no
+validity buffer.
+
+As an example, you could have the following data: ::
+
+    type: Float32
+    [1.0, 1.0, 1.0, 1.0, null, null, 2.0]
+
+In Run-length-encoded form, this could appear as:
+
+::
+
+    * Length: 3, Null count: 2
+    * Accumulated run lengths buffer:
+
+      | Bytes 0-3   | Bytes 4-7   | Bytes 8-11  | Bytes  6-63           |
+      |-------------|-------------|-------------|-----------------------|
+      | 4           | 6           | 7           | unspecified (padding) |
+
+    * Children arrays:
+
+      * values (Float32):
+        * Length: 3, Null count: 1
+        * Validity bitmap buffer:
+
+          | Byte 0 (validity bitmap) | Bytes 1-63            |
+          |--------------------------|-----------------------|
+          | 00000101                 | 0 (padding)           |
+
+        * Values buffer
+
+          | Bytes 0-3   | Bytes 4-7   | Bytes 8-11  | Bytes  6-63           |
+          |-------------|-------------|-------------|-----------------------|
+          | 1.0         | unspecified | 2.0         | unspecified (padding) |
+
+
 Buffer Listing for Each Layout
 ------------------------------
 
@@ -957,7 +1016,7 @@ The ``Buffer`` Flatbuffers value describes the location and size of a
 piece of memory. Generally these are interpreted relative to the
 **encapsulated message format** defined below.
 
-The ``size`` field of ``Buffer`` is not required to account for padding
+The ``size`` field of ``Buffer`` is not required to account for paddingeng-career-mgmt
 bytes. Since this metadata can be used to communicate in-memory pointer
 addresses between libraries, it is recommended to set ``size`` to the actual
 memory size rather than the padded size.