Incorrect Memory Limit description

The [Memory Limit](https://github.com/google/grr-doc/blob/master/admin.adoc#memory-limit) section of the admin page states:

"Default soft limit is 500MB, but GRR should only use about 30MB. Some volatility plugins can use a lot of memory so we try to be generous. Hard limit is double the soft limit. This is configurable from the config file."
- This appears to be true on Windows, but on Linux there is no hard limit implemented.
- The hard limit is [hard-coded](https://github.com/google/grr/blob/master/client/nanny/windows_nanny.cc#L371) and therefore not configurable.