gh-40109: Fix docstring for enumerate_totallyreal_fields_rel
    
This PR fixes the docsting for enumerate_totallyreal_fields_rel.

- Avoids using `d` as discriminant and index of a list.
- Avoids using `n` and `m` for the degree.

Should we clarify this?
> with *absolute* discriminant `d \leq B`;

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preview.
    
URL: #40109
Reported by: Eloi Torrents
Reviewer(s): Frédéric Chapoton

Author: Release Manager

src/sage/rings/number_field/totallyreal_rel.py

@@ -650,9 +650,9 @@ def enumerate_totallyreal_fields_rel(F, m, B, a=[], verbose=0,
 
     ::
 
-        a[d]*x^n + ... + a[0]*x^(n-d)
+        a[k]*x^m + ... + a[0]*x^(m-k)
 
-    if ``length(a) = d+1``, so in particular always ``a[d] = 1``.
+    if ``length(a) = k+1``, so in particular always ``a[k] = 1``.
 
     .. NOTE::