docs: add connect section to Variable metadata

- while here, update the description on setting unit metadata in   Contextual-Variables

Author: ven-k

docs/src/basics/ContextualVariables.md

@@ -48,7 +48,7 @@ for which its arguments must be specified each time it is used. This is useful w
 PDEs for example, where one may need to use `u(t, x)` in the equations, but will
 need to be able to write `u(t, 0.0)` to define a boundary condition at `x = 0`.
 
-## Variable metadata [Experimental/TODO]
+## Variable metadata
 
 In many engineering systems, some variables act like “flows” while others do not.
 For example, in circuit models you have current which flows, and the related
@@ -69,15 +69,17 @@ the metadata. One can get and set metadata by
 ```julia
 julia> @variables x [unit = u"m^3/s"];
 
-julia> hasmetadata(x, Symbolics.option_to_metadata_type(Val(:unit)))
+julia> hasmetadata(x, VariableUnit)
 true
 
-julia> getmetadata(x, Symbolics.option_to_metadata_type(Val(:unit)))
+julia> ModelingToolkit.get_unit(x)
 m³ s⁻¹
 
-julia> x = setmetadata(x, Symbolics.option_to_metadata_type(Val(:unit)), u"m/s")
+julia> x = setmetadata(x, VariableUnit, u"m/s")
 x
 
-julia> getmetadata(x, Symbolics.option_to_metadata_type(Val(:unit)))
+julia> ModelingToolkit.get_unit(x)
 m s⁻¹
 ```
+
+See [Symbolic Metadata](@ref symbolic_metadata) for more details on variable metadata.

docs/src/basics/MTKModel_Connector.md

@@ -156,7 +156,7 @@ using ModelingToolkit
 end
 ```
 
-Variables (as functions of independent variable) are listed out in the definition. These variables can optionally have initial values and metadata like `description`, `connect` and so on. For more details on setting metadata, check out [Symbolic Metadata](@id symbolic_metadata).
+Variables (as functions of independent variable) are listed out in the definition. These variables can optionally have initial values and metadata like `description`, `connect` and so on. For more details on setting metadata, check out [Symbolic Metadata](@ref symbolic_metadata).
 
 Similar to `@mtkmodel`, `@connector` accepts begin blocks of `@components`, `@equations`, `@extend`, `@parameters`, `@structural_parameters`, `@variables`. These keywords mean the same as described above for `@mtkmodel`.
 For example, the following `HydraulicFluid` connector is defined with parameters, variables and equations.

docs/src/basics/Variable_metadata.md

@@ -1,4 +1,4 @@
-# Symbolic metadata
+# [Symbolic Metadata](@id symbolic_metadata)
 
 It is possible to add metadata to symbolic variables, the metadata will be displayed when calling help on a variable.
 
@@ -39,6 +39,22 @@ help?> u
   Symbolics.VariableSource: (:variables, :u)
 ```
 
+## Connect
+
+Variables in connectors can have `connect` metadata which describes the type of connections.
+
+`Flow` is used for variables that represent physical quantities that "flow" ex:
+current in a resistor. These variables sum up to zero in connections.
+
+`Stream` can be specified for variables that flow bi-directionally.
+
+```@example connect
+using ModelingToolkit
+
+@variables t, i(t) [connect = Flow]
+@variables k(t) [connect = Stream]
+```
+
 ## Input or output
 
 Designate a variable as either an input or an output using the following

docs/src/tutorials/acausal_components.md

@@ -10,7 +10,7 @@ component variables. We then simplify this to an ODE by eliminating the
 equalities before solving. Let's see this in action.
 
 !!! note
-
+    
     This tutorial teaches how to build the entire RC circuit from scratch.
     However, to simulate electric components with more ease, check out the
     [ModelingToolkitStandardLibrary.jl](https://docs.sciml.ai/ModelingToolkitStandardLibrary/stable/)