does RST `foo`:idx: actually work?

[timothee]

see context in this commit: 
[https://github.com/nim-lang/Nim/commit/37986d0ba8d83b76e710b352a7da7cce2a5cb157](https://github.com/nim-lang/Nim/commit/37986d0ba8d83b76e710b352a7da7cce2a5cb157)

typetraits.nim defines: 
    
    
    proc supportsCopyMem*(t: typedesc): bool {.magic: "TypeTrait".}
      ## This trait returns true iff the type ``t`` is safe to use for
      ## `copyMem`:idx:. Other languages name a type like these `blob`:idx:.
      ...
    
    
    Run

however searching blob in 
[https://nim-lang.github.io/Nim/typetraits.html#supportsCopyMem%2Ctypedesc](https://nim-lang.github.io/Nim/typetraits.html#supportsCopyMem%2Ctypedesc)
 search bar doesn't point to supportsCopyMem (and doc of supportsCopyMem 
[https://nim-lang.github.io/Nim/typetraits.html#supportsCopyMem%2Ctypedesc](https://nim-lang.github.io/Nim/typetraits.html#supportsCopyMem%2Ctypedesc)
 doesn't highlight blob as being a reference/idx)