[0002] RootSignatures - Fixes to Root Signature Specifiction

[inbelic]

This issue tracks a collection of small issues to the root signature specification that were uncovered as part of testing work.

This will be marked as designing until the below list of notes below is comprehensive.

AC:

• Denote that keywords are case-insensitive, and registers are not (B3 is not valid)
• numDescriptors also allows the value of unbounded inclusive of an unsigned number
• Manual offset must be able to be user-specified and has DESCRIPTOR_RANGE_OFFSET_APPEND enum value
• Revisit the use of NUMBER in the grammar. Many values only accept an unsigned integer and DXC will error if given otherwise
• Document that all parameters (including mandatory) can be specified in any order and can only be specified exactly once
• Double-check the AllowLowTierReservedHwCbLimit root flag, it is not recognized in dxc
• Add validation of the maximum values of offset, register, space, numDescriptors, num32BitConstants