You can install
libstdc++6-4.5-doc to get the manuals for the GNU c++ library, and then
man std_basic_string tells you about the contents of that class.
You may ask: how should I know to look in that particular manpage? Well, what you want is the
-k (keyword) argument to man:
mbp@joy% man -k c_str
std_basic_streambuf (3cxx) - (unknown subject)
std_basic_string (3cxx) - (unknown subject)
std_basic_stringbuf (3cxx) - (unknown subject)
std_basic_stringstream (3cxx) - (unknown subject)
and then you can ask man for one of those particular pages.
Or, of course, if you search the web (more thoroughly than I did), you might be able to find a PDF of the standard library definition.