doc: Periodic spellcheck of the documentation. * doc/interpreter/doccheck/aspell-octave.en.pws: Update Octave-specific Aspell dictionary. * doc/interpreter/contrib.txi, doc/interpreter/errors.txi, doc/interpreter/expr.txi, doc/interpreter/io.txi, doc/interpreter/preface.txi, doc/interpreter/sparse.txi, doc/interpreter/stmt.txi, doc/interpreter/strings.txi, libinterp/corefcn/qz.cc, libinterp/dldfcn/ccolamd.cc, libinterp/dldfcn/fftw.cc, libinterp/interpfcn/graphics.cc, scripts/general/structfun.m, scripts/image/ind2rgb.m, scripts/image/rgb2ind.m, scripts/io/importdata.m, scripts/linear-algebra/duplication_matrix.m, scripts/linear-algebra/housh.m, scripts/miscellaneous/mkoctfile.m, scripts/plot/comet3.m, scripts/plot/gco.m, scripts/plot/legend.m, scripts/plot/print.m, scripts/signal/fftshift.m, scripts/signal/periodogram.m, scripts/sparse/pcg.m, scripts/statistics/tests/f_test_regression.m, scripts/statistics/tests/t_test_regression.m, scripts/strings/strjoin.m, scripts/strings/strsplit.m: Correct spellings or add @nospell macro.

--- a/scripts/signal/fftshift.m
+++ b/scripts/signal/fftshift.m
@@ -24,7 +24,7 @@
 ## center of the vector or matrix.
 ##
 ## If @var{x} is a vector of @math{N} elements corresponding to @math{N}
-## time samples spaced by @math{dt}, then
+## time samples spaced by @nospell{@math{dt}}, then
 ## @code{fftshift (fft (@var{x}))} corresponds to frequencies
 ##
 ## @example
@@ -32,7 +32,7 @@
 ## @end example
 ##
 ## @noindent
-## where @nospell{@math{df}} = 1 / @math{dt}.
+## where @nospell{@math{df} = 1 / @math{dt}}.
 ##
 ## If @var{x} is a matrix, the same holds for rows and columns.  If
 ## @var{x} is an array, then the same holds along each dimension.