doc: Replace "non-zero" with "nonzero" to match existing usage. Replace all occurrences in both documentation and code comments. * doc/interpreter/contrib.txi, doc/interpreter/diagperm.txi, doc/interpreter/external.txi, doc/interpreter/sparse.txi, doc/interpreter/stmt.txi, doc/interpreter/testfun.txi, doc/refcard/refcard.tex, examples/mysparse.c, libinterp/corefcn/balance.cc, libinterp/corefcn/cellfun.cc, libinterp/corefcn/data.cc, libinterp/corefcn/filter.cc, libinterp/corefcn/find.cc, libinterp/corefcn/kron.cc, libinterp/corefcn/ls-mat5.cc, libinterp/corefcn/luinc.cc, libinterp/corefcn/mappers.cc, libinterp/corefcn/oct-fstrm.cc, libinterp/corefcn/oct-fstrm.h, libinterp/corefcn/oct-iostrm.cc, libinterp/corefcn/oct-iostrm.h, libinterp/corefcn/oct-stdstrm.h, libinterp/corefcn/oct-strstrm.h, libinterp/corefcn/spparms.cc, libinterp/corefcn/toplev.cc, libinterp/corefcn/utils.cc, libinterp/dldfcn/symrcm.cc, libinterp/octave-value/ov-bool-mat.cc, liboctave/array/CSparse.cc, liboctave/array/Sparse.cc, liboctave/array/Sparse.h, liboctave/array/dSparse.cc, liboctave/numeric/randmtzig.c, liboctave/operators/Sparse-op-defs.h, scripts/help/get_first_help_sentence.m, scripts/miscellaneous/edit.m, scripts/plot/draw/pie.m, scripts/plot/draw/pie3.m, scripts/sparse/colperm.m, scripts/sparse/nonzeros.m, scripts/sparse/spdiags.m, scripts/sparse/spfun.m, scripts/sparse/spones.m, scripts/sparse/sprand.m, scripts/sparse/sprandn.m, scripts/sparse/sprandsym.m, scripts/sparse/spstats.m, scripts/sparse/svds.m, scripts/special-matrix/gallery.m, scripts/statistics/base/moment.m, scripts/statistics/tests/cor_test.m: Replace "non-zero" with "nonzero" to match existing usage.