Incorrect comments about key length in MemTable

1. ​​Incorrect comment in MemTable::Get​​

```
// entry format is:
// klength varint32
// userkey char[klength] // ← Problem here
// tag uint64
// vlength varint32
// value char[vlength]
```

**Fix**: `userkey char[klength - 8]`.

2. ​​Incorrect comment in MemTable::Add​​

```
// Format of an entry is concatenation of:
// key_size : varint32 of internal_key.size()
// key bytes : char[internal_key.size()] // ← Problem here
// tag : uint64((sequence << 8) | type)
// value_size : varint32 of value.size()
// value bytes : char[value.size()]
```

**Fix**: `key bytes : char[key.size()] `.

Search the C++ sources for MemTable::Get and MemTable::Add, then inspect the nearby entry-format comments. Update the two documented key-length expressions to match the issue, and verify that both comments describe the actual entry layout.

Written by the indexing model from the issue text.