runtime(doc): style: clarify to prefer 2 spaces after a sentence

Signed-off-by: Christian Brabandt <[email protected]>

Author: chrisbra

runtime/doc/helphelp.txt

@@ -1,4 +1,4 @@
-*helphelp.txt*	For Vim version 9.1.  Last change: 2025 Apr 08
+*helphelp.txt*	For Vim version 9.1.  Last change: 2025 Apr 21
 
 
 		  VIM REFERENCE MANUAL    by Bram Moolenaar
@@ -383,6 +383,10 @@ standard Vim help files, except for the first line.  If you are writing a new
 help file it's best to copy one of the existing files and use it as a
 template.
 
+Vim help files generally use 2 spaces after a sentence (since they are written
+using a fixed-width font and that was the prefered style in the 70s/80s), like
+what is described here: https://english.stackexchange.com/a/2602
+
 The first line in a help file should have the following format:
 
 *plugin_name.txt*	{short description of the plugin}