docs book: use A4 format, new builder option format.

docs/conf.py

@@ -193,12 +193,6 @@ htmlhelp_basename = 'borgdoc'
 
 # -- Options for LaTeX output --------------------------------------------------
 
-# The paper size ('letter' or 'a4').
-#latex_paper_size = 'letter'
-
-# The font size ('10pt', '11pt' or '12pt').
-#latex_font_size = '12pt'
-
 # Grouping the document tree into LaTeX files. List of tuples
 # (source start file, target name, title, author, documentclass [howto/manual]).
 latex_documents = [
@@ -210,6 +204,11 @@ latex_documents = [
 # the title page.
 latex_logo = '_static/logo.pdf'
 
+latex_elements = {
+    'papersize': 'a4paper',
+    'pointsize': '10pt',
+}
+
 # For "manual" documents, if this is true, then toplevel headings are parts,
 # not chapters.
 #latex_use_parts = False