Added description of the new fucntionality of erase tool

[SeqLaz]

This adds a new erase tool for lines and polygons.

https://github.com/opengisch/QField/pull/4584

[duke-nyuki]

Task linked: QF-4044 #4584 Brand new geometry editor erase tool functionality

[m-kuhn]

@SeqLaz what do you think about the suggestions by @nirvn ?

[nirvn]

We can merge this.