[Doc] Extended documentation of <is_linear>

Documentation/ProjectFile/prj/processes/process/ComponentTransport/t_is_linear.md

-This flag will enable two optimizations for the ComponentTransport process:
+This flag enables two optimizations for the ComponentTransport process:
 
 1. the global equation systems are assembled only once, in the first timestep,
    and reused subsequently
 2. the non-linear solver will converge in a single iteration in each timestep
 
 \attention
-With this option enabled, OGS will produce correct results only if the process equations are linear.
+With this option enabled, **OGS will produce correct results only if the process equations are linear**.
 OGS will not detect any non-linearities. It is the responsibility of the user to
 ensure that the assembled equation systems are linear, indeed!
+Furthermore, the material properties and parameters used in the process's
+equations must **not carry any time dependence**.