Add synchronisation description to urukul (#4)

Signed-off-by: Egor Savkin <es@m-labs.hk>

src/hw/urukul.md

@@ -12,6 +12,7 @@
     "dds": "<variant>", // ad9910/ad9912
     "ports": [<port num>, <port num>], // second port is optional
     "clk_sel": <clock num>,
+    "synchronization": true/false, // for AD9910 only
     "refclk": <freq>, // for external clock signal
     "pll_en": <0 or 1, default 1> // PLL bypass, to allow higher external clocker frequencies (1e9 for example)
 }
@@ -21,6 +22,16 @@
 
 Check if [SUServo](./suservo.md) is enabled/disabled respective to customer needs. Connect to the clocker source.
 
+### Synchronization
+
+Synchronization option in the JSON refers to the phase synchronization between the outputs, and can be used only on AD9910 variants and
+only with 125 MHz clock source provided from Kasli/Kasli-SoC (may be relayed through the Clocker board).
+The phase sync works only within one Urukul board, though the phase shift between Urukuls may be [predictable](https://github.com/m-labs/artiq/issues/1692#issuecomment-994439589). 
+Even though it is widely-desirable feature, there are drawbacks of this preventing from enabling by default:
+1. The resulting signal is more noisy, which can be observed [previously](https://github.com/sinara-hw/Urukul/issues/64).
+2. Phase sync process takes time and sometimes fails
+3. ???
+
 ## Testing
 
 After running `artiq_sinara_test`: