[1.5.X] Fixed #19348 - Clarified the units of filesizeformat.

Thanks george_edison for the report and Claude Paroz for the patch.

Backport of 63e6cd1bb3 from master

docs/ref/templates/builtins.txt

@@ -1439,6 +1439,14 @@ For example::
 
 If ``value`` is 123456789, the output would be ``117.7 MB``.
 
+.. admonition:: File sizes and SI units
+
+    Strictly speaking, ``filesizeformat`` does not conform to the International
+    System of Units which recommends using KiB, MiB, GiB, etc. when byte sizes
+    are calculated in powers of 1024 (which is the case here). Instead, Django
+    uses traditional unit names (KB, MB, GB, etc.) corresponding to names that
+    are more commonly used.
+
 .. templatefilter:: first
 
 first