Update manual.md

[igor-petruk]

Adding the not that Rust supports manual memory management too. The message here: Rust can do 100% of what C and C++ does, it is not fundamentally limited as some languages with heavy memory management runtimes.

[igor-petruk]

Alternatively, this could be wrapped into the speaker notes.

[igor-petruk]

Alternative 2: add this to the "Memory Management in Rust" section

[mgeisler]

I think manual.md should be devoted to just explaining what 'manual memory management' is, using C as an example language.

Thanks @jiyongp, I agree with that. It's important to keep the slides small and focused — I've noticed in class that the whole course has become a lot more verbose than it was 2-3 months ago. That's in some sense incredible since we cover more nuances now, but it's also a problem since it makes the slides more complicated to show to the students.

[mgeisler]

This slide goes away with #1073 so we can close this now.