Include in the documentation a mention of optional arguments

[Lucas-C]

Currently they are only mentioned as inline comments in the code:
https://github.com/voyages-sncf-technologies/hesperides-jenkins-lib/blob/master/src/com/vsct/dt/hesperides/jenkins/pipelines/Hesperides.groovy#L41

We should mention them in hesperides.txt, either by migrating them there, or by automating their insertion in hesperides.txt (by extrating each corresponding def line from the Hesperides class ?)