[Docu] Update documentation for LinearProperty.

Documentation/ProjectFile/properties/property/LinearProperty/c_LinearProperty.md

 Definition of a linear property:
-\f$y(x) = y_{\textrm{ref}} + m (x - x_{\textrm{ref}})\f$
+\f$y(x) = y_{\textrm{ref}} (1 + m (x - x_{\textrm{ref}})\f$
 where
  - \f$y_{\textrm{ref}}\f$ is a reference value, for instance reference density
- - \f$m\f$ is the slope of the linear relationship
+ - \f$m\f$ is a value influencing the slope of the linear relationship
  - \f$x_{\textrm{ref}}\f$ is a reference condition, for instance reference
-   temperatue
+   temperature

Documentation/ProjectFile/properties/property/LinearProperty/independent_variable/t_slope.md

-The slope \f$m\f$ of the linear relation.
+\f$m\f$ is a value influencing the slope of the linear relation. The slope is
+\f$y_{\mathrm{ref}} m.\f$