message: minor updates to IR documentation
diff: @@ -74,7 +74,11 @@ Names of tensor values flowing through the graph are unique - a particular name
  The list of nodes defining the top-level computation graph MUST be ordered topologically \- that is, if node K follows node N in the graph, none of the data inputs of N may refer to outputs of K.
  +#### Values
  +The representation distinguishes between two kinds of values: attribute values, which are statically known, and runtime values. The type of values permitted in the two cases are different. The permitted types of attribute values are indicated by the enumeration `AttributeType`, while the permitted types of runtime values are described by `TypeProto`.
  +
  +The types of the inputs and outputs of the model must be specified, including the shapes of tensors. While the ranks of input and output tensors are statically specified, the sizes of specific dimensions (axis) may be statically unknown and are indicated so using symbolic identifiers in the shape.
  Built-in Operators and Standard Data Types
  ------------------------------------------
  