Clarify global exports values can employ dots

[mfikes]

See https://github.com/clojure/clojurescript/blob/master/src/main/clojure/cljs/compiler.cljc#L1236-L1247

This implies that if you have something in the global scope, but want to associate a sub-object with the synthetic foreign lib namespace, you can use dots to do so. (The value need not simply name a global variable.)

Some verbiage in the :global-exports explanation at https://clojurescript.org/reference/compiler-options#foreign-libs could be added, clarifying that this is the way things work.

[swannodette]

Another issue where need is now significantly lower due to :target :bundle.