[Attr][Doc] Fix pragma unroll documentation.

There is a contradiction in the #pragma unroll behavior documentation.
It says that specifying `#pragma unroll` without a parameter directs the
loop unroller to attempt to partially unroll the loop if the trip count
is not known at compile time. At the same time later it states that
`#pragma unroll` has identical semantics to `#pragma clang loop unroll(full)`,
which doesn't attempt to unroll partially if the trip count is not known
at compile time.

pragma clang loop unroll(enable):
If unroll(enable) is specified the unroller will attempt to fully unroll
the loop if the trip count is known at compile time. If the fully
unrolled code size is greater than an internal limit the loop will be
partially unrolled up to this limit. If the trip count is not known at
compile time the loop will be partially unrolled with a heuristically
chosen unroll factor.

pragma clang loop unroll(full):
If unroll(full) is specified the unroller will attempt to fully unroll
the loop if the trip count is known at compile time identically to
unroll(enable). However, with unroll(full) the loop will not be unrolled
if the loop count is not known at compile time.

Differential Revision: https://reviews.llvm.org/D136160

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