rust: improved docstring
In the previous wording, rustfmt wanted to cut at the == which is not very
readable in my taste.
Differential Revision: https://phab.mercurial-scm.org/D5414
BasedOnStyle: LLVM
IndentWidth: 8
UseTab: ForIndentation
BreakBeforeBraces: Linux
AllowShortIfStatementsOnASingleLine: false
IndentCaseLabels: false
AllowShortBlocksOnASingleLine: false
AllowShortFunctionsOnASingleLine: false
IncludeCategories:
- Regex: '^<'
Priority: 1
- Regex: '^"'
Priority: 2