gh-126719: Clarify math.fmod docs (#127741)

Co-authored-by: Bénédikt Tran <10796600+picnixz@users.noreply.github.com>

Doc/library/math.rst

@@ -248,7 +248,8 @@ Floating point arithmetic
 
 .. function:: fmod(x, y)
 
-   Return ``fmod(x, y)``, as defined by the platform C library. Note that the
+   Return the floating-point remainder of ``x / y``,
+   as defined by the platform C library function ``fmod(x, y)``. Note that the
    Python expression ``x % y`` may not return the same result.  The intent of the C
    standard is that ``fmod(x, y)`` be exactly (mathematically; to infinite
    precision) equal to ``x - n*y`` for some integer *n* such that the result has