changelog and docs to match nthr-capping done in PR #431; fixes #430

CHANGELOG

@@ -1,6 +1,8 @@
 List of features / changes made / release notes, in reverse chronological order.
 If not stated, FINUFFT is assumed (cuFINUFFT <=1.3 is listed separately).
 
+* CPU plan stage prevents now caps # threads at omp_get_max_threads (being 1
+  for single-thread build); warns if this cap was activated (PR 431)
 * new docs troubleshooting accuracy limitations due to condition number of the
   NUFFT problem.
 * new sanity check on nj and nk (<0 or too big); new err code, tester, doc.

docs/opts.rst

@@ -128,7 +128,7 @@ Diagnostic options
 Algorithm performance options
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 
-**nthreads**: Number of threads to use. This sets the number of threads FINUFFT will use in FFTW, bin-sorting, and spreading/interpolation steps. This number of threads also controls the batch size for vectorized transforms (ie ``ntr>1`` :ref:`here <c>`). Setting ``nthreads=0`` uses all threads available. For repeated small problems it can be advantageous to use a small number, such as 1.
+**nthreads**: Number of threads to use. This is capped at the number of available threads (eg, to prevent misuse of a single-threaded code). It then sets the number of threads FINUFFT will use in FFTW, bin-sorting, and spreading/interpolation steps. This number of threads also controls the batch size for vectorized transforms (ie ``ntr>1`` :ref:`here <c>`). Setting ``nthreads=0`` uses all threads available, usually recommended. However, for repeated small problems it can be advantageous to use a small number, even as small as 1.
 
 **fftw**: FFTW planner flags. This number is simply passed to FFTW's planner;
 the flags are documented `here <http://www.fftw.org/fftw3_doc/Planner-Flags.html#Planner-Flags>`_.