Added missing copyright comments. Corrected links to the documentation on the comments.

doc/explicit_operator_bool.qbk

@@ -1,5 +1,3 @@
-[section:explicit_operator_bool explicit_operator_bool]
-
 [/
  / Copyright (c) 2013 Andrey Semashev
  /
@@ -7,6 +5,8 @@
  / file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
  /]
 
+[section:explicit_operator_bool explicit_operator_bool]
+
 [/=================]
 [simplesect Authors]
 [/=================]