Document checkParamsLength

As a side note, I should probably make those method return boolean.
Step 1 of ICU->boolean: append return statements

I'm going to change these methods to return booleans and have Opcode
throw an exception if relevant in the execute method.
Step 2 of ICU->boolean: use the return values in Opcode.execute
Step 3 of ICU->boolean: remove exception throwing from ICU methods
Document return of ICU.checkParamsLength
Document ICU.checkValidBearing