docs: clarify quantized Add + tanh fusion limitation in TFLite

[Atchayasree03]

This PR documents a known limitation in quantized TFLite conversion where
Add + tanh activations are not fused by the MLIR converter.

Although TANH is defined as a fused activation in the TFLite schema,
only ReLU-based activations are currently fused during quantized conversion.

Related issue: #833

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