In the Filter Function Reference section of GeoServer documentation (http://docs.geoserver.org/latest/en/user/filter/function_reference.html), two fixes might be needed
A. The order of parameters of numberFormat function looks misleading. This function works when the format is set at the first child node and the number is at the second like below.
What the current document is suggesting is that the number is the first parameter and the format second. I think they should be switched.
B. Math function "sin" is not documented whilst "cos" and "tan" are.
The function "sin" (sine) is not present in the Math Function subsection unlike the other trigonometric siblings (cos, tan, asin, acos, atan).
I had a try and confirmed that "sin" is already available at 2.8.2 as a filter function.
It would be nice not to keep the sine function as an easter egg by including it in the documentation.