update docs/PERFORMANCE-NOTES: rm stale mentions of json parsers

follow-up fcdc956

Author: SomberNight

docs/PERFORMANCE-NOTES

@@ -36,10 +36,10 @@ account in the code.
   faster than a new call to len(b)
 
 - The cpython and pypy stdlibs use a Python-based JSON serializer/deserializer,
-  which is stable and easy to use and install, but much slower than
-  system-compiled variants. ElectrumX now looks for either of the popular
-  C-based JSON libraries for Python, python-rapidjson or ujson. If it finds
-  one, it's used instead of the stdlib module for improved performance.
+  which is stable and easy to use and install, but somewhat slower than
+  system-compiled variants. ElectrumX now looks for known popular
+  C/Rust-based JSON libraries for Python. If it finds one,
+  it's used instead of the stdlib module for improved performance.
 
   To install a known-compatible system-compiled JSON lib, install ElectrumX
-  with either the "rapidjson" or "ujson" extras parameter.
+  with the "orjson" extras parameter.