str.lines() docstring: clarify that line endings are not returned

Previously, the str.lines() docstring stated that lines are split at line
endings, but not whether those were returned or not.  This new version of the
docstring states this explicitly, avoiding the need of getting to doctests to
get an answer to this FAQ.

Author: zacchiro

core/src/str/mod.rs

@@ -970,8 +970,10 @@ impl str {
 
     /// An iterator over the lines of a string, as string slices.
     ///
-    /// Lines are ended with either a newline (`\n`) or a carriage return with
-    /// a line feed (`\r\n`).
+    /// Lines are split at line endings that are either newlines (`\n`) or
+    /// sequences of a carriage return followed by a line feed (`\r\n`).
+    ///
+    /// Line terminators are not included in the lines returned by the iterator.
     ///
     /// The final line ending is optional. A string that ends with a final line
     /// ending will return the same lines as an otherwise identical string