document "/&" syntax

[erickt]

@graydon says that a type that is suffixed with /&r describes that this type holds a borrowed pointer from the region r. This needs mentioning in our documentation.

[bblum]

@nikomatsakis

[catamorphism]

This is discussed in the borrowed pointer tutorial already -- see the section "Returning borrowed pointers" -- as well as in the "Pointer types" section of the reference manual ("Borrowed pointers are written &content, or in some cases &f/content for some lifetime-variable f...") If you have suggestions for improvements to the existing doc, feel free to comment or open a new issue :-)