Rollup merge of #77838 - RalfJung:const-fn, r=kennytm

const keyword: brief paragraph on 'const fn'

`const fn` were mentioned in the title, but called "deterministic functions" which is not their main property (though at least currently it is a consequence of being const-evaluable). This adds a brief paragraph discussing them, also in the hopes of clarifying that they do *not* have any effect on run-time uses.

Author: JohnTitor

library/std/src/keyword_docs.rs

@@ -102,7 +102,9 @@ mod break_keyword {}
 
 #[doc(keyword = "const")]
 //
-/// Compile-time constants and deterministic functions.
+/// Compile-time constants and compile-time evaluable functions.
+///
+/// ## Compile-time constants
 ///
 /// Sometimes a certain value is used many times throughout a program, and it can become
 /// inconvenient to copy it over and over. What's more, it's not always possible or desirable to
@@ -145,15 +147,28 @@ mod break_keyword {}
 ///
 /// Constants, like statics, should always be in `SCREAMING_SNAKE_CASE`.
 ///
+/// For more detail on `const`, see the [Rust Book] or the [Reference].
+///
+/// ## Compile-time evaluable functions
+///
+/// The other main use of the `const` keyword is in `const fn`. This marks a function as being
+/// callable in the body of a `const` or `static` item and in array initializers (commonly called
+/// "const contexts"). `const fn` are restricted in the set of operations they can perform, to
+/// ensure that they can be evaluated at compile-time. See the [Reference][const-eval] for more
+/// detail.
+///
+/// Turning a `fn` into a `const fn` has no effect on run-time uses of that function.
+///
+/// ## Other uses of `const`
+///
 /// The `const` keyword is also used in raw pointers in combination with `mut`, as seen in `*const
 /// T` and `*mut T`. More about `const` as used in raw pointers can be read at the Rust docs for the [pointer primitive].
 ///
-/// For more detail on `const`, see the [Rust Book] or the [Reference].
-///
 /// [pointer primitive]: primitive.pointer.html
 /// [Rust Book]:
 /// ../book/ch03-01-variables-and-mutability.html#differences-between-variables-and-constants
 /// [Reference]: ../reference/items/constant-items.html
+/// [const-eval]: ../reference/const_eval.html
 mod const_keyword {}
 
 #[doc(keyword = "continue")]