bpo-18423: Documents limitation for -m flag

[CuriousLearner]

This tries to solve http://bugs.python.org/issue18423 and documents the limitation on -m flag that it only works with leaf-level modules and packages.

[the-knights-who-say-ni]

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[ncoghlan]

The linked bug had already been closed as invalid, so there aren't any updates needed to the documentation for it.

[berkerpeksag]

@ncoghlan should http://bugs.python.org/issue18423 also be closed?

[ncoghlan]

@berkerpeksag Indeed it should, and I've now fixed that oversight (turns out I had clicked through to http://bugs.python.org/issue18422 when I mentioned above that the issue had already been closed)

[ncoghlan]

Added the sprint label, as this PR was submitted at the PyCon Pune 2017 core development sprint.