runtime(doc): link help-writing from write-local-help

fixes: #15862

Signed-off-by: Christian Brabandt <cb@256bit.org>

Author: chrisbra

runtime/doc/usr_51.txt

@@ -352,8 +352,9 @@ was called from a mapping defined with ":noremap".
 DOCUMENTATION						*write-local-help*
 
 It's a good idea to also write some documentation for your plugin.  Especially
-when its behavior can be changed by the user.  See |add-local-help| for how
-they are installed.
+when its behavior can be changed by the user.  See |help-writing| for the
+syntax used by the help files and |add-local-help| for how local help files
+are installed.
 
 Here is a simple example for a plugin help file, called "typecorrect.txt": >