MANUAL.txt: self-contained implies standalone

MANUAL.txt

@@ -772,13 +772,13 @@ Options affecting specific writers
 
 :   Produce a standalone HTML file with no external dependencies, using
     `data:` URIs to incorporate the contents of linked scripts, stylesheets,
-    images, and videos. The resulting file should be "self-contained,"
-    in the sense that it needs no external files and no net access to be
-    displayed properly by a browser. This option works only with HTML output
-    formats, including `html4`, `html5`, `html+lhs`, `html5+lhs`, `s5`,
-    `slidy`, `slideous`, `dzslides`, and `revealjs`. Scripts, images, and
-    stylesheets at absolute URLs will be downloaded; those at relative URLs
-    will be sought relative to the working directory (if the first source
+    images, and videos. Implies `--standalone`. The resulting file should be
+    "self-contained," in the sense that it needs no external files and no net
+    access to be displayed properly by a browser. This option works only with
+    HTML output formats, including `html4`, `html5`, `html+lhs`, `html5+lhs`,
+    `s5`, `slidy`, `slideous`, `dzslides`, and `revealjs`. Scripts, images,
+    and stylesheets at absolute URLs will be downloaded; those at relative
+    URLs will be sought relative to the working directory (if the first source
     file is local) or relative to the base URL (if the first source
     file is remote).  Elements with the attribute
     `data-external="1"` will be left alone; the documents they