Returns None if the first col chars of s contain a non-whitespace char.
Otherwise returns Some(k) where k is first char offset after that leading
whitespace. Note that k may be outside bounds of s.
Makes a doc string more presentable to users.
Used by rustdoc and perhaps other tools, but not by rustc.
A fast conservative estimate on whether the string can contain documentation links.
A pair of square brackets [] must exist in the string, but we only search for the
opening bracket because brackets always go in pairs in practice.