[docs] Refine the description in Set-Like and Map-Like container options.

In "Other Set-Like Container Options":
* Drops the references to C++ TR1 and SGI and hash_set.
* Drops the worry about portability (this was a problem with hash_set, but
std::unordered_set has worked portably since LLVM started depending
on C++11).

It is similar in "Other Map-Like Container Options" section.

Differential Revision: https://reviews.llvm.org/D117858