doc: Periodic spellcheck of documentation. * doc/interpreter/doccheck/aspell-octave.en.pws: Add new words to Octave-specific dictionary. * scripts/miscellaneous/edit.m: Use @nospell macro to stop spellchecking code samples. * scripts/image/imread.m, scripts/image/imwrite.m, scripts/linear-algebra/cond.m, scripts/plot/orient.m: Periodic spellcheck of documentation.