Rollup merge of #72371 - Elrendio:char_documentation, r=steveklabnik

FIX - Char documentation for unexperienced users

This is my first PR on rust and even if I've read [CONTRIBUTING.md](https://github.com/rust-lang/rust/blob/master/CONTRIBUTING.md#pull-requests) I'm ensure everything is perfect. Sorry if I didn't follow the exact procedure.

**What it does:**
- Add an example in the char documentation

**Explanation**
Unexperienced users might not know that punctuation is `Case_Ignorable` and not `Uppercase` and `Lowercase` which mean that when checking if a string is uppercase one might be tempted to write:
```rust
my_string.chars().all(char::is_uppercase)
```

However this will return false for `"HELLO WORLD"` which is not intuitive. Since the function `is_case_ignorable` doesn't exists I believe the correct way to check is:
```rust
!my_string.chars().any(char::is_lowercase)
```

The aim of this example is to prevent unexperienced users to make an error which punctuation chars.

src/libcore/char/methods.rs

@@ -812,8 +812,9 @@ impl char {
     /// assert!(!'A'.is_lowercase());
     /// assert!(!'Δ'.is_lowercase());
     ///
-    /// // The various Chinese scripts do not have case, and so:
+    /// // The various Chinese scripts and punctuation do not have case, and so:
     /// assert!(!'中'.is_lowercase());
+    /// assert!(!' '.is_lowercase());
     /// ```
     #[stable(feature = "rust1", since = "1.0.0")]
     #[inline]
@@ -843,8 +844,9 @@ impl char {
     /// assert!('A'.is_uppercase());
     /// assert!('Δ'.is_uppercase());
     ///
-    /// // The various Chinese scripts do not have case, and so:
+    /// // The various Chinese scripts and punctuation do not have case, and so:
     /// assert!(!'中'.is_uppercase());
+    /// assert!(!' '.is_uppercase());
     /// ```
     #[stable(feature = "rust1", since = "1.0.0")]
     #[inline]