Rollup merge of rust-lang#72773 - Rantanen:is_char_boundary-docs, r=joshtriplett

Fix is_char_boundary documentation

Given the "start _and/or end_" wording in the original, the way I understood it was that the `str::is_char_boundary` method would also return `true` for the last byte in a UTF-8 code point sequence. (Which would have meant that for a string consisting of nothing but 1 and 2 byte UTF-8 code point sequences, it would return nothing but `true`.)

In practice the method returns `true` only for the starting byte of each sequence and the end of the string: [Playground](https://play.rust-lang.org/?version=nightly&mode=debug&edition=2018&gist=e9f5fc4d6bf2f1bf57a75f3c9a180770)

I was also somewhat tempted to remove the _The start and end of the string are considered to be boundaries_, since that's implied by the first sentence, but I decided to avoid bikeshedding over it and left it as it was since it's not wrong in relation to how the method behaves.

Author: RalfJung

src/libcore/str/mod.rs

@@ -2270,12 +2270,11 @@ impl str {
         self.len() == 0
     }
 
-    /// Checks that `index`-th byte lies at the start and/or end of a
-    /// UTF-8 code point sequence.
+    /// Checks that `index`-th byte is the first byte in a UTF-8 code point
+    /// sequence or the end of the string.
     ///
     /// The start and end of the string (when `index == self.len()`) are
-    /// considered to be
-    /// boundaries.
+    /// considered to be boundaries.
     ///
     /// Returns `false` if `index` is greater than `self.len()`.
     ///