Add "Basic Usage" to int min_value and max_value docs

[nvzqz]

This adds "Basic Usage:" to the docs of min_value and max_value, which makes it consistent with docs of other integer methods.

[rust-highfive]

r? @sfackler

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[steveklabnik]

Thanks so much! @bors: r+ rollup

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[bors]

📌 Commit c08a51c has been approved by steveklabnik

[bors]

⌛ Testing commit c08a51c with merge 5477c23a72670d296bf20879c11ac5313bce7f26...

[bors]

💔 Test failed - status-travis

[kennytm]

@bors retry #47002

[nvzqz]

@steveklabnik should all Examples be followed by "Basic Usage:" if they're not otherwise followed by any text?

[bors]

⌛ Testing commit c08a51c with merge bf0ee965ba4a85e6c395ea7fc229e45d2ff3dd28...

[bors]

💔 Test failed - status-travis

[steveklabnik]

@nvzqz https://github.com/rust-lang/rfcs/blob/master/text/1574-more-api-documentation-conventions.md is the guidelines, which don't technically contain them. I actually thought that they did contain this, though, heh. It is a thing we do in stdlib sometimes.