Expands docs

src/analysis/processing/qgsalgorithminterpolatepoint.cpp

@@ -53,7 +53,11 @@ QString QgsInterpolatePointAlgorithm::outputName() const
 
 QString QgsInterpolatePointAlgorithm::shortHelpString() const
 {
-  return QObject::tr( "This algorithm creates a point geometry interpolated at a set distance along line geometries." );
+  return QObject::tr( "This algorithm creates a point geometry interpolated at a set distance along line or curve geometries.\n\n"
+                      "Z and M values are linearly interpolated from existing values.\n\n"
+                      "If a multipart geometry is encountered, only the first part is considered when "
+                      "interpolating the point.\n\n"
+                      "If the specified distance is greater than the curve's length, the resultant feature will have a null geometry." );
 }
 
 QString QgsInterpolatePointAlgorithm::shortDescription() const

src/analysis/processing/qgsalgorithmlinesubstring.cpp

@@ -56,7 +56,9 @@ QString QgsLineSubstringAlgorithm::shortHelpString() const
   return QObject::tr( "This algorithm returns the portion of a line (or curve) which falls "
                       "between the specified start and end distances (measured from the "
                       "beginning of the line).\n\n"
-                      "Z and M values are linearly interpolated from existing values." );
+                      "Z and M values are linearly interpolated from existing values.\n\n"
+                      "If a multipart geometry is encountered, only the first part is considered when "
+                      "calculating the substring." );
 }
 
 QString QgsLineSubstringAlgorithm::shortDescription() const