Document impl_header_lifetime_elision

[ehuss]

Stabilized in 1.31 (all editions), the lifetime elision chapter needs to be updated for impl headers.

• Tracking issue: Tracking issue for lifetime elision for impl headers (feature impl_header_lifetime_elision) rust#15872
• RFC: https://github.com/rust-lang/rfcs/blob/master/text/0141-lifetime-elision.md
• Implementation: support elision in impl headers rust#49251
  • Separation: Extract impl_header_lifetime_elision out of in_band_lifetimes rust#53016
• Stabilization PR: Stabilize impl_header_lifetime_elision in 2015 rust#54778

I looked at updating this, but things have diverged from the RFC, so the new rules are not clear to me.

[Centril]

cc @scottmcm since they stabilized this.