doc: Miscellaneous documentation fixes all over the manual (bug #54288). * embedded.cc, defaults.cc, error.cc, hex2num.cc, load-save.cc, integral2.m, integral3.m, quad2d.m, warning_ids.m, decic.m, lsqnonneg.m, pqpnonneg.m, bicg.m, betaincinv.m: Removal of duplicate words and redundant ')' and ';' characters. Added missing '"', ')' characters. Renamed variables to match other occurrences in docstring. Use '*' instead of 'x' for multiply. Surround variables with @var{}.

--- a/libinterp/corefcn/hex2num.cc	Tue Jul 10 20:57:59 2018 -0700
+++ b/libinterp/corefcn/hex2num.cc	Wed Jul 11 08:40:07 2018 -0700
@@ -294,7 +294,7 @@
 @end group
 @end example
 
-With the optional second argument @qcode{"cell}, return a cell array of
+With the optional second argument @qcode{"cell"}, return a cell array of
 strings instead of a character array.
 @seealso{hex2num, hex2dec, dec2hex}
 @end deftypefn */)