update outdated doc with new example

[mu001999]

update the illegal definition example because we can compile struct Ref<'a, T> { x: &'a T } (play)

[rustbot]

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[hkBst]

@mu001999 just curious, but do you know what changed when to make this well-formed?

[mu001999]

bisect:
commit[0] 2018-09-11: Auto merge of #53873 - nikomatsakis:nll-universe-subtyping-and-pattern-ascription, r=pnkfelix
commit[1] 2018-09-11: Auto merge of #53913 - petrochenkov:biattr4, r=alexcrichton
commit[2] 2018-09-12: Auto merge of #51159 - pacman82:master, r=oli-obk
commit[3] 2018-09-12: Auto merge of #54146 - kennytm:rollup, r=kennytm
commit[4] 2018-09-12: Auto merge of #53793 - toidiu:ak-stabalize, r=nikomatsakis
commit[5] 2018-09-12: Auto merge of #54152 - michaelwoerister:cgu-name-fix, r=alexcrichton
commit[6] 2018-09-12: Auto merge of #53409 - GuillaumeGomez:associated-const-value, r=QuietMisdreavus

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Regression in nightly-2018-09-13
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fetching (via local git) commits from 2f1547c0aa5957b42cc768c00119c6eb7b4262d3 to f2302daef3608c09e2b50193a64611b18ced86f3
opening existing repository at "/home/r0cky/repos/rust"
Found origin remote under name `origin`
refreshing repository at "/home/r0cky/repos/rust"
looking up first commit
looking up second commit
checking that commits are by bors and thus have ci artifacts...
finding bors merge commits
found 7 bors merge commits in the specified range
  commit[0] 2018-09-11: Auto merge of #53873 - nikomatsakis:nll-universe-subtyping-and-pattern-ascription, r=pnkfelix
  commit[1] 2018-09-11: Auto merge of #53913 - petrochenkov:biattr4, r=alexcrichton
  commit[2] 2018-09-12: Auto merge of #51159 - pacman82:master, r=oli-obk
  commit[3] 2018-09-12: Auto merge of #54146 - kennytm:rollup, r=kennytm
  commit[4] 2018-09-12: Auto merge of #53793 - toidiu:ak-stabalize, r=nikomatsakis
  commit[5] 2018-09-12: Auto merge of #54152 - michaelwoerister:cgu-name-fix, r=alexcrichton
  commit[6] 2018-09-12: Auto merge of #53409 - GuillaumeGomez:associated-const-value, r=QuietMisdreavus
ERROR: no CI builds available between 2f1547c0aa5957b42cc768c00119c6eb7b4262d3 and f2302daef3608c09e2b50193a64611b18ced86f3 within last 167 days

[mu001999]

what changed when to make this well-formed?

@hkBst it should be #53793

[hkBst]

src/test/ui/regions/regions-struct-not-wf.rs seems to contain examples of things that are not well-formed

[hkBst]

what changed when to make this well-formed?

@hkBst it should be #53793

Seems that RFC 2093 got stabilized.