Plugin IAP... very little information about how it works

**Your Godot version:**
3.4.1 rc1 mono

**Issue description:**
The plugin has very little information about its operation in the official godot documentation.

The plugin does not indicate that:
- Signal "Connected" is emited when the plugin connects to the Billing service locally on the smartphone. Do not confuse, the emission of the signal has nothing to do with a connection to google servers.

- The first time "QuerySkuDetails" is called, the smartphone needs to have an internet connection for the call to be successful. Subsequent calls to "QuerySkuDetails" will use the cache that is stored when the first call succeeds.
This means that the user should not base the status of the smartphone connection on the result of the call to "QuerySkuDetails".
The user might think "If QuerySkuDetails succeeds, then the smartphone has an internet connection enabled", this is false.

- Connection errors that occur when the player presses the "Buy" button will be reflected by a native window of the android/GooglePlayStore system indicating that "There is no connection available ...", the user can get rid of handling this situation and simply focus on controlling the shopping process.

**URL to the documentation page (if already existing):**
https://docs.godotengine.org/en/stable/tutorials/platform/android_in_app_purchases.html