Reinstate "abstract override"

[jskeet]

The indexer modifiers description at one point included:

The abstract and override modifiers may be used together so that an abstract indexer can override a virtual one.

This should be in 15.6.1 (Methods / General) when modifiers are introduced with rules.

We'll add it in when default interface implementations etc are introduced in C# 8.

[jskeet]

Leaving assigned to Bill to do as part of his work on #681.