Turn off viewcode extension by default

doc/ext/viewcode.rst

@@ -15,6 +15,11 @@ a highlighted version of the source code, and a link will be added to all object
 descriptions that leads to the source code of the described object.  A link back
 from the source to the description will also be inserted.
 
+This extension works only on HTML related builders like ``html``,
+``applehelp``, ``devhelp``, ``htmlhelp``, ``qthelp`` and so on except
+``singlehtml``. By default ``epub`` builder doesn't
+support this extension (see :confval:`viewcode_enable_epub`).
+
 There is an additional config value:
 
 .. confval:: viewcode_import
@@ -34,3 +39,26 @@ There is an additional config value:
       main routine is protected by a ``if __name__ == '__main__'`` condition.
 
    .. versionadded:: 1.3
+
+.. confval:: viewcode_enable_epub
+
+   If this is ``True``, viewcode extension is also enabled even if you use
+   epub builders. This extension generates pages outside toctree, but this
+   is not preferred as epub format.
+
+   Until 1.4.x, this extension is always enabled. If you want to generate
+   epub as same as 1.4.x, you should set ``True``, but epub format checker's
+   score becomes worse.
+
+   The default is ``False``.
+
+   .. versionadded:: 1.5
+
+   .. warning::
+
+      Not all epub readers support pages generated by viewcode extension.
+      These readers ignore links to pages are not under toctree.
+
+      Some reader's rendering result are corrupted and
+      `epubcheck <https://github.com/IDPF/epubcheck>`_'s score
+      becomes worse even if the reader supports.