Updated the HashMap's documentation to include two references to
add_modify.
The first is when the `Entry` API is mentioned at the beginning. I was
hesitant to change the "attack" example (although I believe that it is
perfect example of where `add_modify` should be used) because both uses
work equally, but one is more idiomatic (`add_modify`).
The second is with the `entry` function that is used for the `Entry`
API. The code example was a perfect use for `add_modify`, which is why
it was changed to reflect that.
d3d22e1e66