This PR revises the descriptions of DXIL module flags.
Descriptions such as `D3D11_1_SB_GLOBAL_FLAG_SKIP_OPTIMIZATION` are
referring to Global Flags in DXBC.
DXBC is not a supported backend target, so references to DXBC should not
be present.
There is also confusion with regards to the description of the
`LowPrecisionPresent` DXIL module flag, which currently reads
`D3D11_1_SB_GLOBAL_FLAG_ENABLE_MINIMUM_PRECISION` and implies the use of
minimum-precision to handle 16-bit types.
However this is not true, because both the flags `LowPrecisionPresent`
and `UseNativeLowPrecision` can simultaneously be set in the same DXIL
module, and minimum precision mode is mutually exclusive with native low
precision.
This PR revises the description of the `LowPrecisionPresent` flag to
accurately describe what it represents.
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