Document what a Kempner number is (#201)

docs/src/refs.bib

@@ -24,3 +24,19 @@ @Book{OSCAR
   month         = {8},
   url           = {https://link.springer.com/book/9783031621260}
 }
+
+@Article{MR1501173,
+    author = {Kempner, Aubrey J.},
+     title = {Polynomials and their residue systems},
+   journal = {Trans. Amer. Math. Soc.},
+  fjournal = {Transactions of the American Mathematical Society},
+    volume = {22},
+      year = {1921},
+    number = {2},
+     pages = {240--266},
+      issn = {0002-9947,1088-6850},
+   mrclass = {11C08 (13P99)},
+  mrnumber = {1501173},
+       doi = {10.2307/1989020},
+       url = {https://doi.org/10.2307/1989020},
+}

src/GenericCyclotomics.jl

@@ -60,7 +60,8 @@ end
 @doc raw"""
     kempner(m::Int64)
 
-Return the `m`-th Kempner number.
+Return the minimal non-negative integer `k` such that `k!` is a multiple of `m`. This is called the `m`-th Kempner number.
+Details about the Kempner numbers and how to compute them can be found in [MR1501173](@cite).
 
 # Examples
 ```jldoctest