HTML5 writer: use "figcaption" only for captions nested in a figure.

Third-party applications may (ab)use the "caption" element
for other purposes as a figure caption.

See sphinx-doc/sphinx#8870

git-svn-id: https://svn.code.sf.net/p/docutils/code/trunk/docutils@8624 929543f6-e4f2-0310-98a6-ba3bd3dd1d04

docutils/writers/html5_polyglot/__init__.py

@@ -183,13 +183,15 @@ def visit_authors(self, node):
     def depart_authors(self, node):
         self.depart_docinfo_item()
 
-    # don't add 'caption' class value
+    # use the <figcaption> semantic tag.
     def visit_caption(self, node):
-        self.body.append('<figcaption>\n')
+        if isinstance(node.parent, nodes.figure):
+            self.body.append('<figcaption>\n')
         self.body.append(self.starttag(node, 'p', ''))
 
     def depart_caption(self, node):
         self.body.append('</p>\n')
+        # <figcaption> is closed in depart_figure(), as legend may follow.
 
     # use HTML block-level tags if matching class value found
     supported_block_tags = set(('ins', 'del'))
@@ -305,8 +307,7 @@ def depart_header(self, node):
                                  'bdi', 'del', 'ins', 'mark', 'small',
                                  'b', 'i', 'q', 's', 'u'))
     def visit_inline(self, node):
-        # Use "supported inline tag" as tag name if found in class values
-        # (first: 
+        # Use `supported_inline_tags` if found in class values
         classes = node.get('classes', [])
         tags = [cls for cls in self.supported_inline_tags
                 if cls in classes]