Add documentation for the integral variable pp, refs idaholab#17566

framework/doc/content/source/postprocessors/ElementIntegralVariablePostprocessor.md

@@ -1,4 +1,21 @@
 # ElementIntegralVariablePostprocessor
 
-The ElementIntegralVariablePostprocessor is an intermediate base class that should be derived from for any calculation involing
-the integral of a variable quantity over the whole domain or a portion of the domain.
+The ElementIntegralVariablePostprocessor is an intermediate base class that should be derived from for any calculation involving
+the integral of a variable quantity over the whole domain or a portion of the domain.
+
+The ElementIntegralVariablePostprocessor may be used to compute the volume integral of a variable, based on the quadrature defined
+for this variable. This integral may be restricted to blocks/subdomains on which the variable is defined.
+
+## Example input file
+
+In this input file, we use `ElementIntegralVariablePostprocessor` to compute the integral
+of a variable $u$, solution of a diffusion problem, over each and all of the two subdomains it
+is defined on.
+
+!listing postprocessors/element_integral/element_block_integral_test.i
+
+!syntax parameters /Postprocessors/ElementIntegralVariablePostprocessor
+
+!syntax inputs /Postprocessors/ElementIntegralVariablePostprocessor
+
+!syntax children /Postprocessors/ElementIntegralVariablePostprocessor