Add naming guidance to Azure API Guidelines

[mikekistler]

Add guidance for naming of properties, parameters, etc to the Azure API Guidelines.

We can probably borrow some of the guidance the MS Graph team has written:

https://github.com/microsoft/api-guidelines/blob/vNext/graph/GuidelinesGraph.md#naming

[heaths]

A few common ones come to mind re: dates e.g.,

• createdOn
• updatedOn
• notBefore
• expiresOn (often notAfter)
• confidence/confidenceThreshold (drop "Score")

Some of these were discussed at length early on for our first track 2 GAs like Key Vault, but I think over time have been forgotten. Better to bake that into the swagger development process - or at least into the code gen / SDK review process.

[JeffreyRichter]

For Boolean naming, I like the guidance here

[mikekistler]

This was addressed by #391