Backport documentation improvement.

Issue24898 - Improve str.find documentation.

Simplify str.find explaination as per Georg Brandl's suggestion.

Doc/library/stdtypes.rst

@@ -963,10 +963,9 @@ string functions based on regular expressions.
 
 .. method:: str.find(sub[, start[, end]])
 
-   Return the lowest index in the string where substring *sub* is found, such
-   that *sub* is contained in the slice ``s[start:end]``.  Optional arguments
-   *start* and *end* are interpreted as in slice notation.  Return ``-1`` if
-   *sub* is not found.
+   Return the lowest index in the string where substring *sub* is found within
+   the slice ``s[start:end]``.  Optional arguments *start* and *end* are
+   interpreted as in slice notation.  Return ``-1`` if *sub* is not found.
 
    .. note::