Rewrite recommended demangling for lifetimes using "De Bruijn level".

Author: ehuss

src/doc/rustc/src/symbol-mangling/v0.md

@@ -545,14 +545,14 @@ Indices starting from 1 refer (as de Bruijn indices) to a higher-ranked lifetime
 > **Recommended Demangling**
 >
 > A *lifetime* may be displayed like a Rust lifetime using a single quote.
-> Index 0 should be displayed as `'_`.
->
-> Lifetimes starting from 1 may be translated to single lowercase letters starting with `'a`.
-> Indices over 25 may consider printing the numeric lifetime index as in `'_123`.
 >
+> Index 0 should be displayed as `'_`.
 > Index 0 should not be displayed for lifetimes in a *[ref-type]*, *[mut-ref-type]*, or *[dyn-trait-type]*.
 >
-> Nested binders may consider tracking their indices so that lifetime lettering can start back with `'a` within a nested binder.
+> A lifetime can be displayed by converting the De Bruijn index to a De Bruijn level
+> (level = number of bound lifetimes - index) and selecting a unique name for each level.
+> For example, starting with single lowercase letters such as `'a` for level 0.
+> Levels over 25 may consider printing the numeric lifetime as in `'_123`.
 > See *[binder]* for more on lifetime indexes and ordering.
 
 > Example: