Tidy functions.jl and improve docstrings (#2108)

Co-authored-by: Benoît Legat <benoit.legat@gmail.com>

Author: odow

docs/src/reference/standard_form.md

@@ -12,28 +12,30 @@ DocTestFilters = [r"MathOptInterface|MOI"]
 
 ```@docs
 AbstractFunction
+output_dimension
+constant
+```
+
+## Scalar functions
+
+```@docs
 AbstractScalarFunction
-AbstractVectorFunction
 VariableIndex
-VectorOfVariables
 ScalarAffineTerm
 ScalarAffineFunction
-VectorAffineTerm
-VectorAffineFunction
 ScalarQuadraticTerm
 ScalarQuadraticFunction
-VectorQuadraticTerm
-VectorQuadraticFunction
 ```
 
-### Utilities
+## Vector functions
 
 ```@docs
-output_dimension
-constant(f::Union{ScalarAffineFunction, ScalarQuadraticFunction})
-constant(f::Union{VectorAffineFunction, VectorQuadraticFunction})
-constant(f::VariableIndex, ::Type)
-constant(f::VectorOfVariables, T::Type)
+AbstractVectorFunction
+VectorOfVariables
+VectorAffineTerm
+VectorAffineFunction
+VectorQuadraticTerm
+VectorQuadraticFunction
 ```
 
 ## Sets