[price-service] Document the spy filters in .env.sample (#472)

* document the filters

* better doc

Co-authored-by: Jayant Krishnamurthy <jkrishnamurthy@jumptrading.com>

third_party/pyth/price-service/.env.sample

@@ -1,7 +1,10 @@
 # Local DevNet:
 SPY_SERVICE_HOST=0.0.0.0:7072
 
-## Filters (if provided) should be valid JSON like below:
+# Filters (if provided) should be valid JSON like below:
+# These filters tell the spy to only retrieve messages sent from certain chains/contracts.
+# See the docker-compose.<network>.yaml files in the price-service directory for the appropriate
+# configuration for a testnet/mainnet pyth price-service deployment.
 SPY_SERVICE_FILTERS=[{"chain_id":1,"emitter_address":"71f8dcb863d176e2c420ad6610cf687359612b6fb392e0642b0ca6b1f186aa3b"}]
 
 # Number of seconds to sync with spy to be sure to have latest messages