Doc: Clears up trim_char doc

Closes #18451

Author: gamazeps

src/libcore/str.rs

@@ -1535,7 +1535,7 @@ pub trait StrSlice<'a> {
     /// ```
     fn ends_with(&self, needle: &str) -> bool;
 
-    /// Returns a string with characters that match `to_trim` removed.
+    /// Returns a string with characters that match `to_trim` removed from the left and the right.
     ///
     /// # Arguments
     ///