more standard doc session specification;

--- a/src/Doc/How_to_Prove_it/ROOT	Fri May 02 13:57:57 2014 +0200
+++ /dev/null	Thu Jan 01 00:00:00 1970 +0000
@@ -1,9 +0,0 @@
-session How_to_Prove_it = HOL +
-  options [document = pdf, show_question_marks = false]
-  theories
-    How_to_Prove_it
-  document_files
-    "root.tex"
-    "root.bib"
-    "prelude.tex"
-

--- a/src/Doc/ROOT	Fri May 02 13:57:57 2014 +0200
+++ b/src/Doc/ROOT	Fri May 02 14:15:23 2014 +0200
@@ -73,6 +73,15 @@
     "root.tex"
     "style.sty"
 
+session How_to_Prove_it (doc) in "How_to_Prove_it" = HOL +
+  options [document_variants = "how_to_prove_it", show_question_marks = false]
+  theories
+    How_to_Prove_it
+  document_files
+    "root.tex"
+    "root.bib"
+    "prelude.tex"
+
 session Intro (doc) in "Intro" = Pure +
   options [document_variants = "intro"]
   theories