Update restrictions documentation in restrictions.md

Author: chmerdon

docs/src/restrictions.md

@@ -1,36 +1,37 @@
 # Restrictions
 
-ExtendableFEM.jl provides functionality to apply various restrictions to your finite element problems through the `AbstractRestriction` type system. Restrictions are used to enforce additional constraints on your solution that cannot be easily expressed through standard boundary conditions or the weak formulation.
+ExtendableFEM.jl provides functionality to apply various restrictions to your finite element problems through the `AbstractRestriction` type system. Restrictions enforce additional constraints on your solution that cannot be easily expressed through standard boundary conditions or the weak formulation.
 
 ## Built-in Restrictions
 
-### Mean Value Restriction
+### Linear Functional Restriction
 
-`MeanValueRestriction` allows you to constrain the mean value of a scalar unknown to a specific value. This is particularly useful in problems where solutions are only unique up to a constant.
+`LinearFunctionalRestriction` allows you to constrain a linear functional of a finite element unknown to a specific value. This is useful, for example, for enforcing that the solution has mean zero, or that an integral constraint is satisfied.
 
+Documentation:
 ```@autodocs
 Modules = [ExtendableFEM]
-Pages = ["common_restrictions/mean_value_restriction.jl"]
+Pages = ["common_restrictions/linear_functional_restriction.jl"]
 Order   = [:type, :function]
 ```
 
 #### Example Usage
 ```julia
 # Restrict the mean value of unknown u to 0
-restriction = MeanValueRestriction(u)
+restriction = ZeroMeanValueRestriction(u)
 
 # Restrict the mean value to 1.0 with a specific operator
-restriction = MeanValueRestriction(u, value = 1.0, operator = MyCustomOperator)
+restriction = MassRestriction(u, value = 1.0, operator = MyCustomOperator)
 
-# apply to the problem
+# Assign to the problem
 assign_restriction!(PD, restriction)
 ```
 
 ### Coupled DOFs Restriction
 
-`CoupledDofsRestriction` enables coupling between different degrees of freedom (DOFs) in your system. This is particularly useful for implementing periodic boundary conditions or other constraints where DOFs need to be related to each other. See also `CombineDofs` operator, which does the same by maniplulating the system matrix.
-However, the application of `CoupledDofsRestriction` much faster for larger systems, since the manipulation of the system matrix, as performed by operators, is very expensive.
+`CoupledDofsRestriction` enables coupling between different degrees of freedom (DOFs) in your system. This is particularly useful for implementing periodic boundary conditions or other constraints where DOFs need to be related to each other. Compared to manipulating the system matrix directly via operators (e.g. `CombineDofs`), `CoupledDofsRestriction` is much faster for large systems.
 
+Documentation:
 ```@autodocs
 Modules = [ExtendableFEM]
 Pages = ["common_restrictions/coupled_dofs_restriction.jl"]
@@ -44,3 +45,37 @@ coupling_matrix = get_periodic_coupling_matrix(...)
 restriction = CoupledDofsRestriction(coupling_matrix)
 assign_restriction!(PD, restriction)
 ```
+
+### Boundary Data Restriction
+
+`BoundaryDataRestriction` enforces prescribed boundary values for a finite element unknown. It can be used for both homogeneous (zero) and non-homogeneous boundary conditions, interpolating the boundary data as needed.
+
+Documentation:
+```@autodocs
+Modules = [ExtendableFEM]
+Pages = ["common_restrictions/boundarydata_restriction.jl"]
+Order   = [:type, :function]
+```
+
+#### Example Usage
+```julia
+# Homogeneous boundary data (default)
+restriction = BoundaryDataRestriction(u)
+
+# Inhomogeneous boundary data using a function
+restriction = BoundaryDataRestriction(u, data = (result, qpinfo) -> result .= sin(qpinfo.x[1]))
+
+assign_restriction!(PD, restriction)
+```
+
+## AbstractRestriction API
+
+All restrictions are subtypes of `AbstractRestriction`. The following functions are available for interacting with restrictions:
+
+- `assemble!(restriction, solution, SC; kwargs...)`: Assemble the internal matrices and vectors for the restriction.
+- `restriction_matrix(restriction)`: Get the restriction matrix.
+- `restriction_rhs(restriction)`: Get the right-hand side vector for the restriction.
+- `fixed_dofs(restriction)`: Get the list of fixed degrees of freedom affected by the restriction.
+- `is_timedependent(restriction)`: Returns whether the restriction is time-dependent.
+
+---