Unify and crossref docstrings for GF and finite_field

[lgoettgens]

Resolves oscar-system/Oscar.jl#3669.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 86.06%. Comparing base (03e705a) to head (b027375).
Report is 7 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1806      +/-   ##
==========================================
+ Coverage   85.96%   86.06%   +0.09%     
==========================================
  Files          98       98              
  Lines       36380    36397      +17     
==========================================
+ Hits        31274    31324      +50     
+ Misses       5106     5073      -33     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

[fingolfin]

SingularCI tests fail due to oscar-system/Singular.jl#811

[thofma]

In some other place we rephrased this "algebra generator", see the ?gen:

    gen(L::FqField)

  Return a K-algebra generator a of the finite field L, where K is the base field of L. The element a satisfies
  defining_polyomial(a) == 0.

  Note that this is in general not a multiplicative generator and can be zero, if L/K is an extension of degree
  one.