Hey Krishna,
actually it is documented, but yes, very very hard to find .
Check the "Modeling guide - 8.6 Functions used in Expressions" to find the list of functions to be used in expressions.
For some reasons beyond my understanding the list of expression was maintained twice for the "Expression editor" and for the CE_CALC function and apparently the lists had not been kept in sync.
I hinted the documentation team to this, so this will be addressed.
Cheers,
Lars