[docs] The STL "binary search" has a non-obvious name.

std::lower_bound is the canonical "binary search" in the STL
(std::binary_search generally is not what you want). The name actually
makes a lot of sense (and also has a beautiful symmetry with the
std::upper_bound algorithm). The name is nonetheless non-obvious.

Also, remove mention of "radix search". It's not even clear how that
would work in the context of a sorted vector. AFAIK "radix search" only
makes sense when you have a trie-like data structure.

llvm-svn: 178376