Confusion regarding fmt.rs {:.*} syntax

[leonardinius]

See IRC discussion starting from https://botbot.me/mozilla/rust/2015-03-01/?msg=33094439&page=16

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tl;dr;

• See http://doc.rust-lang.org/std/fmt/index.html#argument-types
• It introduces {:.*} syntax without proper explanation what it is.
• It's confusing, some think it's regexp, other think (e.g. me) it's not
• It does not specify behavior and doesn't provide minimal samples.

It might be that some kind of explanation similar to https://botbot.me/mozilla/rust/msg/33094764/ is required; as well as at least few examples, such as https://botbot.me/mozilla/rust/msg/33094776/, as well as this https://botbot.me/mozilla/rust/msg/33094947/

I 'm afraid I didn't completely grok examples featuring $ from the IRC conversation, but I think they should be explained as well.

[theemathas]

Another documentation problem in fmt: How is width defined for unicode strings?

Current behavior: println!("a{:5}b", "n͈̰̎") outputs an͈̰̎ b

[theemathas]

Related to #22646