Improve functions documentation

[fadjer]

In the 2.4 documentation you do not speak about arithmetic operations, and when we read this part of the documentation we suppose that we must write custom functions for that:

http://docs.graylog.org/en/2.4/pages/pipelines/rules.html

There are two different types of actions:
Function calls
Variable assignments
....

Could you please improve function documentation and detail the existence and the use of arithmetic operator like in the example just-after:

https://github.com/Graylog2/graylog-plugin-pipeline-processor/blob/2.4.0/plugin/src/test/resources/org/graylog/plugins/pipelineprocessor/parser/arithmetic.txt

Many Thanks