GH-46642: [Format] Add footnote clarifying REE layout has O(log n) random access (#46643)

### Rationale for this change

Resolves #46642.

### What changes are included in this PR?

- Updated columnar format doc

### Are these changes tested?

Yes, rendered locally.

### Are there any user-facing changes?

No.
* GitHub Issue: #46642

Authored-by: Bryce Mecum <[email protected]>
Signed-off-by: Bryce Mecum <[email protected]>

Author: amoeba

docs/source/format/Columnar.rst

@@ -40,7 +40,7 @@ while reading this document.
 The columnar format has some key features:
 
 * Data adjacency for sequential access (scans)
-* O(1) (constant-time) random access
+* O(1) (constant-time) random access [#f1]_
 * SIMD and vectorization-friendly
 * Relocatable without "pointer swizzling", allowing for true zero-copy
   access in shared memory
@@ -52,6 +52,9 @@ data representation and serialization details; issues such as
 coordinating mutation of data structures are left to be handled by
 implementations.
 
+.. [#f1] Except for the :ref:`run-end-encoded-layout` where random access is
+    O(log n).
+
 Terminology
 ===========