Make docstring of gen(::FqField) less ambiguous

src/flint/fq_default_extended.jl

@@ -146,11 +146,13 @@ end
 ################################################################################
 
 @doc raw"""
-    gen(a::FqField)
+    gen(L::FqField)
 
-Return the algebra generator of the finite field over its base field.
+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.
+Note that this is in general not a multiplicative generator and can be zero, if
+$L/K$ is an extension of degree one.
 """
 function gen(L::FqField)
   # should not be cached (for in place stuff etc)