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/doc/interpreter/strings.txi
+++ b/doc/interpreter/strings.txi
@@ -96,7 +96,7 @@
 Represents a literal single-quote character, @samp{'}.
 
 @item \0
-Represents the ``nul'' character, control-@@, ASCII code 0.
+Represents the null character, control-@@, ASCII code 0.
 
 @item \a
 Represents the ``alert'' character, control-g, ASCII code 7.
@@ -158,8 +158,8 @@
 @section Character Arrays
 
 The string representation used by Octave is an array of characters, so
-internally the string "dddddddddd" is actually a row vector of length 10
-containing the value 100 in all places (100 is the ASCII code of "d").  This
+internally the string @nospell{"dddddddddd"} is actually a row vector of length
+10 containing the value 100 in all places (100 is the ASCII code of "d").  This
 lends itself to the obvious generalization to character matrices.  Using a
 matrix of characters, it is possible to represent a collection of same-length
 strings in one variable.  The convention used in Octave is that each row in a