These changes bring it closer to other standard library documentation
and, in particular, `std::fs::canonicalize`, which it will often be
compared with.
* Add `# Platform-specific behavior` section, with content moved from
Examples section.
* Create `# Errors` section.
* Phrase error description to allow future platforms to have new
syntactic errors, rather than only emptiness.
* Add missing commas.
* Indent example code 4 spaces.
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