docs(prt): update migration guide and release notes (MODFLOW-ORG#2109)

Add more detail about the exit_solve_tolerance PRP variable and its caveats.

Author: wpbonelli

doc/ReleaseNotes/develop.tex

@@ -54,7 +54,7 @@
 		\item With a flow model using the Newton formulation, the PRT model could crash upon a particle's entry into a dry cell. This has been fixed.
 		\item With a flow model using the Newton formulation, the PRT model could enter an endless loop upon a particle's entry to a dry cell if that cell contains a boundary package (e.g. a pumping well). Where the particle should be captured and terminate, it would instead be passed back and forth between the cell bottom and the top of the cell below. To avoid this, particles are forbidden from backtracking (reentering the previous cell) within the same time step.
 		\item The PRT model now allows more control over vertical particle motion in dry conditions. In addition to the existing DRAPE option, which controls release-time behavior, the PRP package now provides a DRY\_TRACKING\_METHOD option which configures how dry particles (particles in dry cells, or above the water table in partially saturated cells) behave at tracking time. This option is relevant only when the Newton formulation is used, in which case dry cells remain active; otherwise, dry cells are inactive and particles will terminate. See the MF6IO document for a detailed explanation of DRY\_TRACKING\_METHOD.
-		\item The PRT model's Particle Release Point (PRP) package now provides an option EXIT\_SOLVE\_TOLERANCE which configures the tolerance to use when solving for a particle's exit location from an unstructured grid cell. This value is only used for the generalized (ternary) tracking method on vertex grids. A value of 0.00001 is set by default. This value works well for many problems, but the value that strikes the best balance between accuracy and runtime is problem-dependent.
+		\item The PRT model's Particle Release Point (PRP) package now provides an option EXIT\_SOLVE\_TOLERANCE which configures the tolerance to use when solving for a particle's exit location from a triangular subcell of an unstructured grid cell. This value is only used for the generalized (ternary) tracking method on vertex grids. A value of 0.00001 is set by default. This value works well for many problems, but the value that strikes the best balance between accuracy and runtime is problem-dependent.
 	\end{itemize}
 
 	%\underline{INTERNAL FLOW PACKAGES}

src/Solution/ParticleTracker/mf6_6_0_prt_migration_guide.md

@@ -22,9 +22,11 @@ Rather than describing bug fixes and new features separately, this document is s
 
 ## Exit solve tolerance
 
-The PRT model's Particle Release Point (PRP) package now provides an option `EXIT_SOLVE_TOLERANCE` which configures the tolerance to use when solving for a particle's exit location from an unstructured grid cell. This value is only used for the generalized (ternary) tracking method on vertex grids.
-
-A value of 0.00001 is set by default. This value works well for many problems, but the value that strikes the best balance between accuracy and runtime is problem-dependent.
+The PRT model's Particle Release Point (PRP) package now provides an option `EXIT_SOLVE_TOLERANCE` which configures the tolerance to use when solving for a particle's exit location from a triangular subcell of an unstructured grid cell. This value is only used for the generalized (ternary) tracking method on vertex grids.
+ 
+A value of 0.00001 is set by default. This value works well for many problems, but the value that strikes the best balance between accuracy and speed is problem-dependent.
+ 
+**Note**: A tolerance that is too large can result in a particle that is close to a subcell boundary and moving nearly parallel to that boundary to terminate prematurely. If a particle is suspected to have terminated prematurely in a model based on a DISV grid, tightening EXIT_SOLVE_TOLERANCE down (say, to 1e-10) may resolve the issue.
 
 ## Release time selection