[patch] Add a section about implementation-defined behavior to the user's manual

[Nico Weber]

Hi,

a coworker asked me if clang documents how it handles right shifts of
signed numbers today. As far as I can tell there's no documentation
for this, but I found PR11272 about documenting implementation-defined
behavior. The bug suggests just linking to gcc's documentation as a
first step, which is what the attached patch does. This isn't quite
correct (for example, clang supports UCNs without any flags), but is
correct for most things. People can fill in more details over time.

Is this ok?

Nico
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