[doc] Tags documentation of DeactivatedSubdomain.

Documentation/ProjectFile/prj/process_variables/process_variable/deactivated_subdomains/deactivated_subdomain/i_deactivated_subdomain.md

-It defines a set of deactivated subdomains, which includes  a time interval and
- material IDs.
+\copydoc ProcessLib::DeactivatedSubdomain

Documentation/ProjectFile/prj/process_variables/process_variable/deactivated_subdomains/deactivated_subdomain/line_segment/i_line_segment.md

+\copydoc ProcessLib::DeactivatedSubdomain::line_segment

Documentation/ProjectFile/prj/process_variables/process_variable/deactivated_subdomains/deactivated_subdomain/line_segment/t_end.md

+Endpoint of a line segment.

Documentation/ProjectFile/prj/process_variables/process_variable/deactivated_subdomains/deactivated_subdomain/line_segment/t_start.md

+Starting point of the line segment.

Documentation/ProjectFile/prj/process_variables/process_variable/deactivated_subdomains/deactivated_subdomain/t_time_curve.md

+A curve for time dependent element deactivation. This allows fine graded control
+over the elements being deactivated compared to the time interval. The curve,
+referenced by name here, is used to map the time to position along the line
+segment.

Documentation/ProjectFile/prj/process_variables/process_variable/deactivated_subdomains/deactivated_subdomain/t_time_interval.md

-The time interval of a set of deactivated subdomains.
-
-For its input syntax, please also see:
-\copydoc ogs_file_param__prj__time_loop__processes__process__time_interval

Documentation/ProjectFile/prj/process_variables/process_variable/deactivated_subdomains/deactivated_subdomain/time_interval/i_time_interval.md

+Time interval for subdomain deactivation. Contrary to a time curve the
+deactivation happens at once as soon as the start time is reached until the end
+time.

Documentation/ProjectFile/prj/process_variables/process_variable/deactivated_subdomains/deactivated_subdomain/time_interval/t_end.md

+End time for domain deactivation.

Documentation/ProjectFile/prj/process_variables/process_variable/deactivated_subdomains/deactivated_subdomain/time_interval/t_start.md

+Start time for domain deactivation.

Documentation/ProjectFile/prj/process_variables/process_variable/deactivated_subdomains/i_deactivated_subdomains.md

-It defines sets of deactivated subdomains, each of which contains a time
- interval and material IDs. When the time is in the time interval of a set of
- deactivated subdomains, the elements in the deactivated subdomains are excluded
- from the local and global assembly, and also from the computation of the
- secondary variables.
-
-Note: the nodes at the borders among active subdomains and inactive subdomains
- are included through the entire computations.
+Definition of subdomains for time dependent deactivation.

ProcessLib/DeactivatedSubdomain.h

@@ -38,6 +38,25 @@ struct DeactivatedSubdomainMesh
     std::vector<MeshLib::Node*> const inner_nodes;
 };
 
+/// Time depend subdomain deactivation.
+///
+/// Subdomain deactivation is space and time dependent.
+/// The spatial extent of deactivated elements is defined through a set of
+/// material ids.
+/// Additionally a line segment separates through its normal plane and position
+/// on the line segment an active and inactive part.
+///
+/// The subdomain can be deactivated at once using a time interval.
+/// For fine grained control a time curve can be specified.
+/// It maps current time to position on the line segment.
+/// Elements, which center points lie left of this position are deactivated.
+///
+/// \internal
+/// The deactivated elements are excluded from the assembly, pre and post call,
+/// secondary variables computation and the like. To keep the size of the global
+/// linear equation system artificial Dirichlet boundary conditions are applied
+/// on the interior of the deactivated subdomain. The nodes on the border
+/// between the active and inactive elements are not affected.
 struct DeactivatedSubdomain
 {
     DeactivatedSubdomain(