Clarify safety of `std::unique_ptr`

[frederick-vs-ja]

resolve #282

Description

Clarify under which conditions unique_ptr guarantees safety, because it can't introduce absolute safety.

Change List

• Add description regarding the non-copyability of unique_ptr

Reference

• std::unique_ptr<T,Deleter>::unique_ptr - cppreference.com
• std::unique_ptr<T,Deleter>::operator= - cppreference.com
• [unique.ptr.single.ctor]
• [unique.ptr.single.asgn]
• [unique.ptr.runtime.ctor]
• [unique.ptr.runtime.asgn]

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说明

澄清 unique_ptr 在何种情况下保证安全，因为它不可能带来绝对的安全。

变化箱单

• 增加了 unique_ptr 不可复制相关的说明

参考文献

• std::unique_ptr<T,Deleter>::unique_ptr - cppreference.com
• std::unique_ptr<T,Deleter>::operator= - cppreference.com
• [unique.ptr.single.ctor]
• [unique.ptr.single.asgn]
• [unique.ptr.runtime.ctor]
• [unique.ptr.runtime.asgn]