docs: Reword str::strip_prefix and strip_suffix a bit

"Some is returned with <some value>" is an awkward construction.
The use of the passive voice is a bit odd, and doesn't seem like the
house style.

So say instead "returns X, wrapped in `Some`", for which there is some
other precedent in stdlib.

Instead of repeating "with the prefix removed", say "after the
prefix".  This is a bit clearer that the original is not modified.

Signed-off-by: Ian Jackson <ijackson@chiark.greenend.org.uk>

Author: ijackson

library/core/src/str/mod.rs

@@ -1964,11 +1964,12 @@ impl str {
 
     /// Returns a string slice with the prefix removed.
     ///
-    /// If the string starts with the pattern `prefix`, `Some` is returned with the substring where
-    /// the prefix is removed. Unlike `trim_start_matches`, this method removes the prefix exactly
-    /// once.
+    /// If the string starts with the pattern `prefix`, returns
+    /// substring after the prefix, wrapped in `Some`.
+    /// Unlike `trim_start_matches`, this method removes the
+    /// prefix exactly once.
     ///
-    /// If the string does not start with `prefix`, `None` is returned.
+    /// If the string does not start with `prefix`, returns `None`.
     ///
     /// The [pattern] can be a `&str`, [`char`], a slice of [`char`]s, or a
     /// function or closure that determines if a character matches.
@@ -1992,11 +1993,12 @@ impl str {
 
     /// Returns a string slice with the suffix removed.
     ///
-    /// If the string ends with the pattern `suffix`, `Some` is returned with the substring where
-    /// the suffix is removed. Unlike `trim_end_matches`, this method removes the suffix exactly
-    /// once.
+    /// If the string ends with the pattern `suffix`, returns the
+    /// substring before the suffix, wrapped in `Some`.
+    /// Unlike `trim_end_matches`, this method removes the
+    /// suffix exactly once.
     ///
-    /// If the string does not end with `suffix`, `None` is returned.
+    /// If the string does not end with `suffix`, returns `None`.
     ///
     /// The [pattern] can be a `&str`, [`char`], a slice of [`char`]s, or a
     /// function or closure that determines if a character matches.