Improve the documentation about extending the parser for k4run

[jmcarcell]

BEGINRELEASENOTES

• Improve the documentation about extending the parser for k4run

ENDRELEASENOTES

Some minor syntax changes and a comment about the [0] that I typically forget when writing from memory.

[m-fila]

Could you also add usage example from cli? For instance something like

The example steering file can then be executed as follows:

k4run example_steering_file.py --trackingOnly --some-other-args 

[jmcarcell]

Done