Rollup merge of #22989 - laijs:fix_FromStr_bool_comment, r=alexcrichton
Rollup merge of #22989 - laijs:fix_FromStr_bool_comment, r=alexcrichton

 Fix the return type in the comments.

An old commit 082bfde41217 (\"Fallout of std::str stabilization\") removed
the example of FromStr::from_str(), this commit adds it back. But
the example of StrExt::parse() is still kept with an additinal note.

Signed-off-by: Lai Jiangshan <4c9eb49378914b743bdd32292c04df29b820d73e@cn.fujitsu.com>

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