Add section on ~[], DVec, etc to the tutorial

[nikomatsakis]

It's certainly going to be a common question!

To be clear: I mean the borrowed pointer tutorial

[nickdesaulniers]

Scratch the part about dvecs as per Issue #4985

[kud1ing]

DVec are removed now: #5279 (comment)

[graydon]

I am not sure this requires a specific bug since appending to ~[] seems to work pretty easily now; in any case this is not 0.6-blocking.

[sanxiyn]

I think this can be closed now.