enhance fetchPut documentation

[snirye]

While working with the hash map's fetchPut function, I encountered a double free issue. Here's the relevant code:

const key = try fm.alloc.dupe(u8, uri);
const f = try fm.alloc.create(file);

f.* = file{
    .text = text,
    .tree = tree,
};

const prev_value = try fm.files_map.fetchPut(
    key,
    f,
);

if (prev_value) |e| {
    fm.destoryFile(e.value);
    fm.alloc.free(e.key);
}

Later, when destroying the map, I used:

while (it.next()) |f| {
    fm.destoryFile(f.value_ptr.*);
    fm.alloc.free(f.key_ptr.*);
}

This caused a double free error because it's not clearly documented that the key passed to fetchPut is not used when an entry with the same key already exists. The original key is retained, and the new one must be freed manually in that case. I missed this detail, leading to the key being freed twice: once in the overwrite logic, and again during map destruction.