doc: Periodic grammarcheck of documentation. * func.txi, io.txi, error.cc, file-io.cc, regexp.cc, utils.cc, variables.cc, dlmwrite.m, strread.m, textread.m, textscan.m: Use @xbackslashchar to produce a backslash and resolve differences between Info viewer and pdf view. * warning_ids.m: Use @option around command line options. * qmr.m: Use @nospell macro for names in citation. * genpropdoc.m: Eliminate trailing space to closing '}'.

--- a/libinterp/corefcn/utils.cc
+++ b/libinterp/corefcn/utils.cc
@@ -1365,7 +1365,7 @@
 \n\
 Implementation Note: Strings are first converted to double values before the\n\
 checks for valid indices are made.  Unless a string contains the NULL\n\
-character \"\\0\", it will always be a valid index.\n\
+character @nospell{\"\\0\"}, it will always be a valid index.\n\
 @end deftypefn")
 {
   octave_value retval;