Document `rustc_index::vec::IndexVec`

[pierwill]

rustc_index::vec::IndexVec is used in 103 files in the compiler, yet it lacks documentation.

[mfrw]

@rustbot claim

[pnkfelix]

@mfrw happy to see you claim this. I figure I'll just note: If you're wondering about the motivation, it wouldn't hurt to review the commit that introduced the IndexVec and Idx abstractions: bc1eb67

[mfrw]

@mfrw happy to see you claim this. I figure I'll just note: If you're wondering about the motivation, it wouldn't hurt to review the commit that introduced the IndexVec and Idx abstractions: bc1eb67

Appreciate the pointer :)
Thank you

[JKAnderson409]

@mfrw Can I claim this one?

[mfrw]

@mfrw Can I claim this one?

Please do.. I am not getting the time at the moment

[JKAnderson409]

Thanks! @rustbot claim