Rollup merge of #40935 - donniebishop:str_boilerplate_docs, r=steveklabnik

Modify str Structs descriptions

References #29375. Modified descriptions of multiple structs to be more in line with structs found under [`std::iter`](https://doc.rust-lang.org/std/iter/#structs), such as [`Chain`](https://doc.rust-lang.org/std/iter/struct.Chain.html) and [`Enumerate`](https://doc.rust-lang.org/std/iter/struct.Enumerate.html)

src/libcollections/str.rs

@@ -133,9 +133,15 @@ fn connect(&self, sep: &str) -> String {
     }
 }
 
-/// External iterator for a string's UTF-16 code units.
+/// An iterator of [`u16`] over the string encoded as UTF-16.
 ///
-/// For use with the `std::iter` module.
+/// [`u16`]: ../../std/primitive.u16.html
+///
+/// This struct is created by the [`encode_utf16`] method on [`str`].
+/// See its documentation for more.
+///
+/// [`encode_utf16`]: ../../std/primitive.str.html#method.encode_utf16
+/// [`str`]: ../../std/primitive.str.html
 #[derive(Clone)]
 #[stable(feature = "encode_utf16", since = "1.8.0")]
 pub struct EncodeUtf16<'a> {

src/libcore/str/mod.rs

@@ -369,11 +369,15 @@ fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
 Section: Iterators
 */
 
-/// Iterator for the char (representing *Unicode Scalar Values*) of a string.
+/// An iterator over the [`char`]s of a string slice.
 ///
-/// Created with the method [`chars`].
+/// [`char`]: ../../std/primitive.char.html
+///
+/// This struct is created by the [`chars`] method on [`str`].
+/// See its documentation for more.
 ///
 /// [`chars`]: ../../std/primitive.str.html#method.chars
+/// [`str`]: ../../std/primitive.str.html
 #[derive(Clone, Debug)]
 #[stable(feature = "rust1", since = "1.0.0")]
 pub struct Chars<'a> {
@@ -553,7 +557,15 @@ pub fn as_str(&self) -> &'a str {
     }
 }
 
-/// Iterator for a string's characters and their byte offsets.
+/// An iterator over the [`char`]s of a string slice, and their positions.
+///
+/// [`char`]: ../../std/primitive.char.html
+///
+/// This struct is created by the [`char_indices`] method on [`str`].
+/// See its documentation for more.
+///
+/// [`char_indices`]: ../../std/primitive.str.html#method.char_indices
+/// [`str`]: ../../std/primitive.str.html
 #[derive(Clone, Debug)]
 #[stable(feature = "rust1", since = "1.0.0")]
 pub struct CharIndices<'a> {
@@ -625,12 +637,13 @@ pub fn as_str(&self) -> &'a str {
     }
 }
 
-/// External iterator for a string's bytes.
-/// Use with the `std::iter` module.
+/// An iterator over the bytes of a string slice.
 ///
-/// Created with the method [`bytes`].
+/// This struct is created by the [`bytes`] method on [`str`].
+/// See its documentation for more.
 ///
 /// [`bytes`]: ../../std/primitive.str.html#method.bytes
+/// [`str`]: ../../std/primitive.str.html
 #[stable(feature = "rust1", since = "1.0.0")]
 #[derive(Clone, Debug)]
 pub struct Bytes<'a>(Cloned<slice::Iter<'a, u8>>);
@@ -1161,9 +1174,13 @@ fn next_back(&mut self) -> Option<&'a str>
     delegate double ended;
 }
 
-/// Created with the method [`lines`].
+/// An iterator over the lines of a string, as string slices.
+///
+/// This struct is created with the [`lines`] method on [`str`].
+/// See its documentation for more.
 ///
 /// [`lines`]: ../../std/primitive.str.html#method.lines
+/// [`str`]: ../../std/primitive.str.html
 #[stable(feature = "rust1", since = "1.0.0")]
 #[derive(Clone, Debug)]
 pub struct Lines<'a>(Map<SplitTerminator<'a, char>, LinesAnyMap>);

src/libstd_unicode/u_str.rs

@@ -19,6 +19,12 @@
 
 /// An iterator over the non-whitespace substrings of a string,
 /// separated by any amount of whitespace.
+///
+/// This struct is created by the [`split_whitespace`] method on [`str`].
+/// See its documentation for more.
+///
+/// [`split_whitespace`]: ../../std/primitive.str.html#method.split_whitespace
+/// [`str`]: ../../std/primitive.str.html
 #[stable(feature = "split_whitespace", since = "1.1.0")]
 pub struct SplitWhitespace<'a> {
     inner: Filter<Split<'a, fn(char) -> bool>, fn(&&str) -> bool>,