Add paragraph about `number_of_` to the styleguide.

[lgoettgens]

As layed out in https://hackmd.io/nRnyfrSxTVe5CzXidB1cBQ?both#number-of-%E2%80%A6 and implemented in https://github.com/Nemocas/AbstractAlgebra.jl/pull/1553, https://github.com/Nemocas/Nemo.jl/pull/1624, https://github.com/thofma/Hecke.jl/pull/1364, https://github.com/oscar-system/Oscar.jl/pull/3272.

Thanks to @lkastner for pointing out that the style guide change was missed.

[codecov[bot]]

Codecov Report

Merging #3305 (b03e04a) into master (070219e) will increase coverage by 0.39%. Report is 88 commits behind head on master. The diff coverage is n/a.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3305      +/-   ##
==========================================
+ Coverage   81.60%   81.99%   +0.39%     
==========================================
  Files         546      560      +14     
  Lines       73504    74791    +1287     
==========================================
+ Hits        59980    61323    +1343     
+ Misses      13524    13468      -56     

see 123 files with indirect coverage changes

[lgoettgens]

I accepted the suggestion by @benlorenz to be able to add comments to specific things.

[lgoettgens]

To settle this discussion once and for all, I would like all of @fieker @fingolfin @micjoswig @thofma to comment on this PR. Once all of them approve, this is good to get merged, and we can point people to the style guide.

[fieker]

wair for @fingolfin

[thofma]

Fine with me