Clarify meaning of "set flag" in cmdstan-guide

State explicitly that setting `STAN_THREADS`, `STAN_MPI`,
and `STAN_OPENCL` to any value, including falsy values,
turns the corresponding feature on.

Fix stan-dev/cmdstan#1293

src/cmdstan-guide/parallelization.qmd

@@ -21,7 +21,9 @@ on how to rewrite Stan models to use these functions see [Stan's User guide chap
 Once a model is rewritten to use the above-mentioned functions, the model
 must be compiled with the `STAN_THREADS` makefile flag. The flag can be
 supplied in the `make` call but we recommend writing the flag to the
-`make/local` file.
+`make/local` file. If the `STAN_THREADS` flag is set to any non-empty value,
+threads will be enabled.
+
 
 An example of the contents of `make/local` to enable threading with TBB:
 
@@ -77,7 +79,7 @@ implementations are [MPICH](https://www.mpich.org/) and [OpenMPI](https://www.op
 Once a model is rewritten to use `map_rect`, additional makefile flags
 must be written to the `make/local`. These are:
 
-- `STAN_MPI`: Enables the use of MPI with Stan if `true`.
+- `STAN_MPI`: Enables the use of MPI with Stan if set.
 - `CXX`: The name of the MPI C++ compiler wrapper. Typically `mpicxx`.
 - `TBB_CXX_TYPE`: The C++ compiler the MPI wrapper wraps. Typically `gcc` on Linux and `clang` on macOS.
 
@@ -158,8 +160,8 @@ Follow Intel's install instructions given [here](https://software.intel.com/cont
 ### Compiling
 
 In order to enable the OpenCL backend the model
-must be compiled with the `STAN_OPENCL` makefile flag. The flag can be
-supplied in the `make` call but we recommend writing the flag to the
+must be compiled with the `STAN_OPENCL` makefile flag set (to any non-empty value).
+The flag can be supplied in the `make` call but we recommend writing the flag to the
 `make/local` file.
 
 An example of the contents of `make/local` to enable parallelization
@@ -169,7 +171,7 @@ with OpenCL:
 STAN_OPENCL=true
 ```
 
-If you are using OpenCL with an integrated GPU you also need to add the `INTEGRATED_OPENCL` flag, as the sharing of memory between CPU and GPU is slightly different with integrated graphics:
+If you are using OpenCL with an integrated GPU you also need to set the `INTEGRATED_OPENCL` flag, as the sharing of memory between CPU and GPU is slightly different with integrated graphics:
 
 ```
 INTEGRATED_OPENCL=true