doc: Periodic spell check of documentation. * aspell-octave.en.pws: Add new words to custom Octave dictionary. * plot.txi: Correct typo "informations" to "information". * data.cc (Fdiag): Use superdiagonal and subdiagonal instead of hyphenated versions. * tril.cc (Ftril): Use superdiagonal and subdiagonal instead of hyphenated versions. * vech.m: Use superdiagonal instead of supradiagonal. * surfnorm.m: Correct typo "returne" to "returned". * periodogram.m: Use "one-sided" rather than "onesided", "two-sided" rather than "twosided".