Amend documentation of shift operators in perlop.pod.

[sisyphus]

The perlop.pod documentation for "Shift Operators" implied that "all bits would fall off" if the size of the bit-shift was greater than the bitsize of the integer.
AFAICT, "all bits fall off" if the size of the bit-shift is greater than or equal to the bitsize of the integer.
So I've made that correction.

There was also some waffle about what C does. I've reduced that to the pre-existing statement This is unlike in C, where shifting by too many bits is undefined. because I think the rest is irrelevant.

And there was also the incorrect statement:

print 20 << 40;  # 5120 on 32-bit machines,

which I've changed to:

print 20 << 32;  # 0 on 32-bit machines,

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• This set of changes does not require a perldelta entry.

[khwilliamson]

s/integer/the &/

[khwilliamson]

lgtm other than the nit