[docs] [issue26213] Document BUILD_*_UNPACK opcodes

[Serhiy Storchaka]

Serhiy Storchaka added the comment:

At the same time the semantic of the BUILD_MAP opcode was changed. However the documentation was not updated and currently it doesn't match the implementation. This caused an issue in third-party projects that operate with bytecode.

See also issue28810 for documenting bytecode changes in 3.6.

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priority: low -> high
type:  -> behavior
versions: +Python 3.5, Python 3.7

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<http://bugs.python.org/issue26213>
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