It´s not very hard to know how which constants are available.
1 - Write the name of the procedure, like if you were going to call it on the code.
2 - Right-click that name and select "Find Declaration"
You will be taken to it´s declaration
3 - Right click the type you want to know.
This will work for set or enumerated types.
This is probably possible to be implemented via a plugin or on the IDE.
For constants passed as integer, then there is no way for the IDE to know which constants are available.
If you really want this, please, send a patch =)