In general, I don’t find the documentation is that bad. In general again, the information is “somewhere” in there. The main problem I see is a lack of uniformity, making it difficult to find that “somewhere”. As you write in your question, the docs were written by different people, and a different times, and that’s probably the cause for this lack of a common structure.
From the java and cppreference links you cite in example, I infer that you are more interested in the synopsis of the interface than in “tutorial” or “motivation” stuff. For shared_ptr, does http://www.boost.org/doc/libs/1_40_0/libs/smart_ptr/shared_ptr.htm#Synopsis provide what you’re after?
For some libs, the “test” and “example” directories under libs/<library_name> are useful.
You may post your questions, comments and suggestions on the boost users and/or documentation mailing lists. From what I see there, specific documentation improvement suggestions are normally welcomed by the library maintainers.