Docs: Add two new faq for the selected site usage and site id convention

Question.1: Should I use 'AppPrefs' or 'SelectedSiteRepository' as the
source of truth on a selected site?
Question.2: Should I use 'id', 'siteId', 'siteLocalId', 'siteRemoteId'
or any other naming convention?

Author: ParaskP7

docs/faq.md

@@ -3,4 +3,35 @@
 #### I can't build/test/package the project because of a `PermGen space` error.
 
 Create a `gradle.properties` file in the project root directory with the
-following: `org.gradle.jvmargs=-XX:MaxPermSize=1024m`.
+following: `org.gradle.jvmargs=-XX:MaxPermSize=1024m`.
+
+#### Should I use `AppPrefs` or `SelectedSiteRepository` as the source of truth on a selected site?
+
+The short answer is to always use the `SelectedSiteRepository`, which is the in-memory mechanism.
+This should always be your goto class when you want to work with the selected site instance, your
+source of truth.
+
+Getting the selected site from `AppPrefs`, which is the persistence mechanism, should only be
+limited to specific usages. Below is a complete list of those:
+
+- `WordPress.mUpdateSelectedSite`: Which runs in a background task and updates the site information.
+- `WordPress.sanitizeMediaUploadStateForSite()`: Which is being trigger from `onCreate(...)`and
+  `onAppComesFromBackground()`.
+- `UploadService,onMediaUploaded(...)`: Which ensures that the handlers have already received and
+  processed the incoming `OnMediaUploaded` event.
+- `WPMainActivity.initSelectedSite()`: Which is being trigger from multiple place within
+  `WPMainActivity` itself, including but not limited to `onCreate(...)`, `onResume(...)` and
+  `onActivityResult(...)`.
+
+#### Should I use `id`, `siteId`, `siteLocalId`, `siteRemoteId` or any other naming convention?
+
+The `SiteModel.mId` is the local id, which is actually an auto-increment database index that gets
+generated when sites are being persisted locally. However, this id is getting reset when users
+log-out and as such it will NOT be the same for a specific site. As such, when working with such an
+`id` you should always name it as `siteLocalId` to avoid any confusion.
+
+The `SimeModel.mSiteId`, is the remote id, which comes from the backend, and it is generated during
+site creation. Then, it gets persisted within the local database, next to the local id. This id is
+NOT getting reset when users log-out and as such it will always be the same for a specific site, no
+matter what. As such, when working with such an `siteId` you should always name it as `siteRemoteId`
+to avoid any confusion.