Close #8326: Rename master_doc to root_doc

To describe the purpose more accurately, the `master_doc` is now renamed
to `root_doc`.  The old name is still available.  But it is recommeneded
to use new one from now on.

doc/_static/conf.py.txt

@@ -43,7 +43,7 @@ source_suffix = '.rst'
 # source_encoding = 'utf-8-sig'
 
 # The master toctree document.
-master_doc = 'index'
+root_doc = 'index'
 
 # General information about the project.
 project = u'test'
@@ -252,7 +252,7 @@ latex_elements = {
 # (source start file, target name, title,
 #  author, documentclass [howto, manual, or own class]).
 latex_documents = [
-    (master_doc, 'test.tex', u'test Documentation',
+    (root_doc, 'test.tex', u'test Documentation',
      u'test', 'manual'),
 ]
 
@@ -283,7 +283,7 @@ latex_documents = [
 # One entry per manual page. List of tuples
 # (source start file, name, description, authors, manual section).
 man_pages = [
-    (master_doc, 'test', u'test Documentation',
+    (root_doc, 'test', u'test Documentation',
      [author], 1)
 ]
 
@@ -298,7 +298,7 @@ man_pages = [
 # (source start file, target name, title, author,
 #  dir menu entry, description, category)
 texinfo_documents = [
-    (master_doc, 'test', u'test Documentation',
+    (root_doc, 'test', u'test Documentation',
      author, 'test', 'One line description of project.',
      'Miscellaneous'),
 ]