[mlir][dxsa] Сonvention for documenting op semantics in DXSA descriptions

[tagolog]

(Raised by @hvdijk in #167.)

Problem

Op descriptions across the DXSA dialect are written in noticeably different styles. The spec documents instructions inconsistently, and
that inconsistency has leaked into our .td files. We should agree on a single convention and apply it dialect-wide.

Examples of the current divergence

22.10.1 add

Description:    Component-wise add.
Operation:      dest = src0 + src1

22.11.1 and

Description:    Bitwise and.
Operation:      Component-wise logical AND of each pair of 32-bit values from src0 and src1. 32-bit results placed in dest.

Open questions to decide

• Formula vs prose vs both for ops.
• Operand order/direction: descriptions don't say which operand is the destination vs the sources (raised by @hvdijk in [mlir][dxsa] Add and instruction #167). Do we state it explicitly, or rely on the $dst, $lhs, $rhs convention?