Non-lexical lifetimes (RFC 2025, RFC 2094)

[matthewjasper]

See rust-lang/rust#44928. The reference currently has almost no documentation of borrowck

• Document lifetimes (I'll try to get to this soon eventually)
• Document borrowck in enough detail for the NLL changes to matter. The main changes are that lifetimes don't have to correspond to lexical scopes, and that 'outlives' is now will eventually be defined as happening at a point in the control flow graph, rather than globally.
• Use in examples if there are any.
• Move ad hoc documentation of borrowck requirements out of the expressions chapter.

[matthewjasper]

cc rust-lang/rust#54129, rust-lang/rust#46901