tutorial: rewrite the section vectors/strings

This section desperately needs to be expanded, but removing the
misleading/incorrect information is a priority.

Managed vectors/strings are not covered, as they are feature-gated and
are only a micro-optimization to avoid double-indirection.

Closes #6882
auto merge of #10354 : thestinger/rust/vector, r=huonw

This section desperately needs to be expanded, but removing the
misleading/incorrect information is a priority.

Managed vectors/strings are not covered, as they are feature-gated and
are only a micro-optimization to avoid double-indirection.

Closes #6882