Update WebTwain_Buffer.md

Author: lunaaaw

info/api/WebTwain_Buffer.md

@@ -1805,7 +1805,7 @@ Action types include
 - `add`: New pages are added to the buffer.
 - `remove`: The existing pages are removed.
 - `modify`: The existing pages are modified.
-- `shift`: The existing pages are reordered.
+- `shift`: The existing pages are reordered or the selected pages are changed.
 - `filter`: The existing pages are filtered by a tag.
 
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