Make it clear that CacheControl is a function that mounts an adapter

The camel-case notation suggests that CacheControl is a class. Given the docs terminology, according to which CacheControl is a "wrapper", I expect that CacheControl returns an object that wraps a session.

But CacheControl is neither a class nor it returns a wrapper object. It's a function which mounts an HTTPAdapter to the session and returns the session itself. It's a function with side-effects that could even return None.

My problem with it is that it's not even innocently misleading, because if the user has already mounted an HTTPAdapter for "http://" and "https://", CacheControl will overwrite that adapter without the user even realizing what happened because he/she was convinced by the naming convention and the docs that CacheControl is a wrapper.

Consider:

• switching to a name that reflects what's actually happening, e.g. mount_cache_adapter;
• warning the user that any eventual adapter for those prefixes will be overwritten.

Start at the CacheControl function and inspect how it mounts an HTTPAdapter on the session for the http:// and https:// prefixes. Decide whether the requested API naming change or an overwrite warning is intended, then verify that the chosen behavior makes the adapter replacement explicit.