examples: fix annoying off-by-one errors

* examples/extexi: Since we issue #lines only at the beginning of
@example, leave empty line when removing content (such as @comment
lines), otherwise the lines that follow have incorrect source line
location.  This leaves ugly empty lines, but they are removed when you
tidy the output for the end user: sequences of \n are mapped to at
most two sucessive \n.

examples/extexi

@@ -60,7 +60,11 @@ sub normalize($)
 {
   local ($_) = @_;
 
-  s/^\@(c |comment|dots|end (ignore|group)|ignore|group).*\n//mg;
+  # If we just remove this lines, then the compiler's tracking of
+  # #lines is broken.  Leave lines that that accepted by all our tools
+  # (including flex, hence the leading space), and that will be easy
+  # to remove (see the Make examples-unline recipe).
+  s{^\@(c |comment|dots|end (ignore|group)|ignore|group).*}{ /**/}mg;
   s/\@value\{VERSION\}/$ENV{VERSION}/g;
   s/^\@(error|result)\{\}//mg;
   s/\@([{}@])/$1/g;