Various changes to documentation framework.

- Reorganize top-level 'doc' Makefile target so submodules can easily
  add their own doc-generating routines to it.  e.g. the Bro project
  makes a placeholder 'doc' target, then adds 'restdoc', 'sphinxdoc';
  later Broccoli can add it's own target as a dependency for generating
  API docs.

- Fixed generated docs for BIFs not being organized under a base/
  subdirectory like the original source files.

- Fixed documentation style for function parameters not applying to
  functions declared as record fields.

- Misc. script documentation tweaks to address warnings given by Sphinx.

scripts/base/frameworks/logging/main.bro

@@ -52,7 +52,7 @@ export {
 		## If not given, all entries are recorded.
 		##
 		## rec: An instance of the streams's ``columns`` type with its
-		## fields set to the values to logged.
+		##      fields set to the values to logged.
 		##
 		## Returns: True if the entry is to be recorded.
 		pred: function(rec: any): bool &optional;