If something’s not specified, general C# guidelines apply: C# Coding Conventions and General Naming Conventions. The talk How To Design A Good API And Why It Matters is an evergreen as well as the .NET Framework Design Guidelines Digest.
This page and many others here are part of the open-source, portable package of Orchard Core goodies: the Orchard Dojo Library for Orchard Core. The Library is freely accessible by anyone; just go to its repository to download its full content (you can even download the textual content of the Library as a big concatenated document in HTML), file issues or fork it! You can even edit it online on Bitbucket, just click on the Edit link in the upper right corner!
Note that the Library also has an Orchard 1.x version, check it out here!