Rephrase paragraph describing how dependency resolution works

- Clarify that pip is incrementally building up this information.
- Clarify that pip has to download files, which contain the dependency information.
- Hint at the fact that this is a recursive process.

docs/html/topics/dependency-resolution.md

@@ -11,10 +11,12 @@ When a user does a `pip install` (e.g. `pip install tea`), pip needs to work
 out the package's dependencies (e.g. `spoon`, `hot-water`, `tea-leaves` etc.)
 and what the versions of each of those dependencies it should install.
 
-pip does not have all the information it needs to work out the dependencies of
-the requested packages. Over the course of the dependency resolution process,
-pip needs to download distributions of the package, which can then provide
-the list of dependencies that the package requires.
+At the start of a `pip install` run, pip does not have all the dependency
+information of the requested packages. It needs to work out the dependencies
+of the requested packages, the dependencies of those dependencies, and so on.
+Over the course of the dependency resolution process, pip will need to download
+distribution files of the packages which are used to get the dependencies of a
+package.
 
 ## Backtracking