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/io/importdata.m
+++ b/scripts/io/importdata.m
@@ -43,9 +43,9 @@
 ## Different file types are supported:
 ##
 ## @itemize
-## @item Ascii table
+## @item ASCII table
 ##
-## Importing ascii table using the specified number of header rows and
+## Importing ASCII table using the specified number of header rows and
 ## the specified delimiter.
 ##
 ## @item Image file
@@ -54,7 +54,7 @@
 ##
 ## @item Spreadsheet files (depending on external software)
 ##
-## @item Wav file
+## @item WAV file
 ##
 ## @end itemize
 ##