Add additive groups to the documentation

[rbeezer]

Add the relatively new additive group documentation from #6449 into the reference manual and verify that it builds properly.

[rbeezer]

Patch:

1) Adds the modules for additive abelian groups to the documentation, which builds just fine.

2) Some minor touchups and additions to the documentation.

3) I removed the note about the output format, as this seems to have been fixed.

4) I added some doctests about creating elements, since this had me very confused when I created #9695.

[cremona]

Good job (especially the extra notes). Applies fine to 4.5.3.alpha1, builds with no problems, and the output looks good.