Merge pull request #1689 from SciML/fb/paramopt

Clarify usage intent of `tunable` metadata

Author: baggepinnen

docs/src/basics/Variable_metadata.md

@@ -34,7 +34,7 @@ isdisturbance(u)
 ```
 
 ## Mark parameter as tunable
-Indicate that a parameter can be automatically tuned by automatic control tuning apps.
+Indicate that a parameter can be automatically tuned by parameter optimization or automatic control tuning apps.
 
 ```@example metadata
 @parameters Kp [tunable=true]