bpo-45584: Clarify the meaning of truncate in documentation

While floor/ceil 's documentation are very precise, `truncate` was not explained. I actually had to search online to understand the difference between `truncate` and `floor` (admittedly, once I remembered that numbers are signed, and that floating numbers actually uses a bit for negation symbol instead of two complement, it became obvious)

I assume that I won't be the only user having this trouble, especially for people whose mother tongue is not English. So I suggest adding a clarification to help make what should be obvious to most actually explicit

Author: Arthur-Milchior

Doc/library/math.rst

@@ -299,8 +299,10 @@ Number-theoretic and representation functions
 .. function:: trunc(x)
 
    Return the :class:`~numbers.Real` value *x* truncated to an
-   :class:`~numbers.Integral` (usually an integer). Delegates to
-   :meth:`x.__trunc__() <object.__trunc__>`.
+   :class:`~numbers.Integral` (usually an integer). Truncating *x* means
+   removing the digits after the decimal separator, hence rounding toward 0. It
+   is equivalent to floor and ceil for positive and negative numbers
+   respectively.  Delegates to :meth:`x.__trunc__() <object.__trunc__>`.
 
 .. function:: ulp(x)