[docs] [issue27604] More details about `-O` flag

[R. David Murray]

R. David Murray added the comment:

It might be better to reduce redundancy (ie: minimize the places that would need changing) by creating a cross link from the option description to the technical details in veryhigh.rst.  You could make the existing words 'basic optimizations' the link text.

Doc changes will be made to 3.5 and 3.6.  I've adjusted the versions accordingly.  A single patch against 3.5 will be merged into 3.6 by the committer.

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versions:  -Python 3.3, Python 3.4

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