GC could do with more consolidated intro user documentation

i.e. not the devdocs where the details are discussed already, but near the start of the manual, mentioning things like:
- that julia has a GC (and characteristics?)
- that it is expected that your system has swap space, but if it doesn't the arg `--heap-size-hint=<size>` can be used to lower the amount of memory to consume.
- multithreading setup and recommendations

etc.?
 