docs: Correct default behavior for missing unique keys

[dajiaohuang]

Correction

The DataTable configuration paragraph says a missing unique key returns null by default. The generated default FindBy*** instead throws KeyNotFoundException, as also described later in MemoryDatabase/RangeView. Returning null requires IsReturnNullIfKeyNotFound = true.

Correct that paragraph to match the default and the existing opt-in setting, without changing runtime behavior.

Validation

• Existing MemoryKeyMemoryTest.Unique verifies missing unique keys throw.
• Existing generator NoGeneratorOptions / FullOptions tests verify default throwing versus the opt-in null mode.
• An isolated .NET 9 application installing NuGet MasterMemory 3.0.4 with the README Person schema confirms missing FindByPersonId throws KeyNotFoundException.
• Baseline runtime tests (28) and generator tests (7) passed. Documentation diff/whitespace checked; no code or generated artifacts changed.