Historical version. PostgreSQL 9.4 has reached end of support. 2020-02-13. See the current manual.
41.3. Data Values in PL/Tcl
The argument values supplied to a PL/Tcl function's code are simply the input arguments converted to text form (just as if they had been displayed by a SELECT statement). Conversely, the return command will accept any string that is acceptable input format for the function's declared return type. So, within the PL/Tcl function, all values are just text strings.
Report a documentation issue
Read the upstream documentation. For corrections, first check the current manual.