[llvm] Document non-trivial `llvm::DenseMapBase` member functions

There are (now?) some non-trivial memberfunctions in `DenseMapBase` that would be good to document, possibly adding a code sample.

From looking at https://llvm.org/doxygen/classllvm_1_1DenseMap.html, examples include:
1) `try_emplace`
2) `insert_or_assign`
3) `emplace_or_assign`
4) `lookup_or`
5) `operator[]` - should specify what happens if the key isn't yet in the map.