I have not found any official language specifications for C# 6 and 7 on Microsoft or ECMA site, but here are few links.
While Microsoft's documentation for C# 6 and 7 at these links is succinct, the StackOverflow pages have lots of examples, details and cover gotchas.
For C# 7
What's new in C# 7 on Microsoft website.
For C# 6
What's New in C# 6 on Microsoft website.
For C# 5 and older versions
If someone stumbles upon this question looking for C# version 5, 4 or 2 specifications, Jon Skeet at C# in Depth
page has put together a nice collection of all the specifications.
P.S. I am intentionally pasting an image snapshot of his page (instead of contents in text format) so I don't take the traffic away from his page.
