tools: kconfig2html: Explain blank-line guard

Explain why the guard runs before checking for a continuation marker.

Signed-off-by: Old-Ding <35417409+Old-Ding@users.noreply.github.com>

tools/kconfig2html.c

@@ -702,6 +702,8 @@ static char *read_line(FILE *stream)
           g_line[len] = '\0';
         }
 
+      /* A blank line has no continuation marker to inspect. */
+
       if (len == 0)
         {
           g_lnptr = g_line;