Cleanup lifetime guide
Clean pointers guide
get over bold text madness, changes per PR, brought the "returning pointers" section back to pointers guide
fix mostly grammar per PR comments
auto merge of #14387 : alan-andrade/rust/remove_managed_boxes_and_gc, r=brson

My main goals were:
- be clear when we talk about "references" and "pointers"
- remove Managed boxes completely and the concept of GC.

https://github.com/mozilla/rust/issues/13987