runtime(doc): clarify the effect of 'startofline' option

fixes: #15794

Signed-off-by: Christian Brabandt <cb@256bit.org>
diff --git a/runtime/doc/options.txt b/runtime/doc/options.txt
index d1a0691..f0844e2 100644
--- a/runtime/doc/options.txt
+++ b/runtime/doc/options.txt
@@ -7789,7 +7789,8 @@
 	non-blank of the line.  When off the cursor is kept in the same column
 	(if possible).  This applies to the commands:
 	- CTRL-D, CTRL-U, CTRL-B, CTRL-F, "G", "H", "M", "L", "gg"
-	- "d", "<<" and ">>" with a linewise operator
+	- "d", "<<", "==" and ">>" with a linewise operator
+	  (|operator-resulting-pos|)
 	- "%" with a count
 	- buffer changing commands (CTRL-^, :bnext, :bNext, etc.)
 	- Ex commands that only has a line number, e.g., ":25" or ":+".