Update "libs" reference page to reflect modern use of term

[puredanger]

The https://clojure.org/reference/libs page is an old page and has a lot of useful information on it, but uses "lib" as a term to mean a Clojure namespace, which matches many 1.0-era docstrings. However, in modern use, "lib" is more commonly used to refer to a whole library, made up of many Clojure namespaces. This disconnect is confusing and/or jarring depending on your experience level with Clojure.

Should we update this page to clarify, merge with the https://clojure.org/reference/namespaces page, or something else?