Rollup merge of rust-lang#55377 - goffrie:patch-2, r=joshtriplett

Slight copy-editing for `std::cell::Cell` docs

Hopefully this is a bit more precise and also more correct English.

Author: kennytm

src/libcore/cell.rs

@@ -207,8 +207,8 @@ use ptr;
 ///
 /// # Examples
 ///
-/// Here you can see how using `Cell<T>` allows to use mutable field inside
-/// immutable struct (which is also called 'interior mutability').
+/// In this example, you can see that `Cell<T>` enables mutation inside an
+/// immutable struct. In other words, it enables "interior mutability".
 ///
 /// ```
 /// use std::cell::Cell;
@@ -225,10 +225,11 @@ use ptr;
 ///
 /// let new_value = 100;
 ///
-/// // ERROR, because my_struct is immutable
+/// // ERROR: `my_struct` is immutable
 /// // my_struct.regular_field = new_value;
 ///
-/// // WORKS, although `my_struct` is immutable, field `special_field` is mutable because it is Cell
+/// // WORKS: although `my_struct` is immutable, `special_field` is a `Cell`,
+/// // which can always be mutated
 /// my_struct.special_field.set(new_value);
 /// assert_eq!(my_struct.special_field.get(), new_value);
 /// ```