Document how developers can specify availability for their projects

[d-ronnqvist]

There are 4 different configurations that relate to platform availability

• the individual availability of a symbol
• availability specified with a @Available directive
• default availability specified in the documentation catalog's Info.plist
• current platform information passed on the command line as --name=Something,version=1.2.3,beta=true

We should add a user facing article that describe how these configurations relate and how the developer can use them to display availability information in their documentation.