update c++ docs on decimal aggregates

docs/source/cpp/compute.rst

@@ -171,8 +171,8 @@ To avoid exhaustively listing supported types, the tables below use a number
 of general type categories:
 
 * "Numeric": Integer types (Int8, etc.) and Floating-point types (Float32,
-  Float64, sometimes Float16).  Some functions also accept Decimal128 and
-  Decimal256 input.
+  Float64, sometimes Float16).  Some functions also accept
+  Decimal32/64/128/256 input.
 
 * "Temporal": Date types (Date32, Date64), Time types (Time32, Time64),
   Timestamp, Duration, Interval.
@@ -256,7 +256,9 @@ the input to a single output value.
   is always -1, regardless of whether there are nulls in the input.
 
 * \(4) For decimal inputs, the resulting decimal will have the same
-  precision and scale. The result is rounded away from zero.
+  scale and the maximum precision for its width. For instance, an array
+  of ``decimal128(3, 2)`` will return a ``decimal128(38, 2)`` scalar.
+  The result is rounded away from zero.
 
 * \(5) Output is a ``{"min": input type, "max": input type}`` Struct.
 
@@ -270,8 +272,10 @@ the input to a single output value.
   Note that the output can have less than *N* elements if the input has
   less than *N* distinct values.
 
-* \(7) Output is Int64, UInt64, Float64, or Decimal128/256, depending on the
-  input type.
+* \(7) Output is Int64, UInt64, Float64, or Decimal32/64/128/256, depending
+  on the input type. For decimals, the precision is increased to the maximum
+  precision for the type's width. For instance, an array of
+  ``decimal128(3, 2)`` will return a ``decimal128(38, 2)`` scalar.
 
 * \(8) Output is Float64 or input type, depending on QuantileOptions.