doc:style: do not use multiple empty lines

Enable LINE_SPACING but only check multiple blank lines now

Change-Id: I332d4d414a04eec8fc54b49d416a954d30592219
Signed-off-by: Mark Zhuang <mark.zhuang@spacemit.com>
Reviewed-on: https://review.openocd.org/c/openocd/+/8365
Reviewed-by: zapb <dev@zapb.de>
Reviewed-by: Jan Matyas <jan.matyas@codasip.com>
Tested-by: jenkins
Reviewed-by: Antonio Borneo <borneo.antonio@gmail.com>

doc/manual/style.txt

@@ -45,7 +45,7 @@ OpenOCD project.
 - use TAB characters for indentation; do NOT use spaces.
 - displayed TAB width is 4 characters.
 - use Unix line endings ('\\n'); do NOT use DOS endings ('\\r\\n')
-- limit adjacent empty lines to at most two (2).
+- do NOT use multiple empty lines.
 - remove any trailing empty lines at the end of source files
 - do not "comment out" code from the tree nor put it within a block
   @code