org-manual: Inline single-sentence footnotes

* doc/Documentation_Standards.org (org-manual.org specific
conventions): Document that we prefer to use inline footnote
definitions when a footnote consists of a single sentence.  This is to
reduce footnote re-numbering in diffs when a new footnote is added to
the manual.
* doc/org-manual.org: Inline single-sentence footnotes.  Renumber the
rest.

doc/Documentation_Standards.org

@@ -112,6 +112,9 @@ version of the manual.
 
 - Use example blocks for Org syntax instead of "begin_src org".
 
+- Prefer inline footnote definition when a footnote consists of a
+  single sentence.
+
 - Internal links to headlines always start with a star.
 
 - Tags, node properties, are not shown with the surrounding colons.