Auto merge of #60817 - ecstatic-morse:issue-60779, r=Centril

Add stubs to keyword docs

Resolves #60779.

This commit gives each stable keyword a short entry in the "Keywords" section in the docs for `std`. The newly added entries are only a single line each and contain the main purpose of the keyword. I changed some of the existing summary lines for consistency's sake. Each line is either an imperative ("name the type of a trait object" for `dyn`), or an object ("An abstract data type" for `enum`). I tried to avoid using the keyword itself or the word "keyword" in the summary.

Later commits can flesh out each keyword with an example for each context in which it can appear as well as a link to the appropriate part of the rust book.

**edit:**
Here's the list of keywords and summaries (sans formatting) to ease reviewing. I'll try to keep this up to date as I make changes:

keyword | summary
-- | --
Self | The implementing type within a `trait` or `impl` block, or the current type within a type definition.
as | Cast between types, or rename an import.
async | ExperimentalReturn a Future instead of blocking the current thread.
await | ExperimentalSuspend execution until the result of a Future is ready.
break | Exit early from a loop.
const | Compile-time constants and deterministic functions.
continue | Skip to the next iteration of a loop.
crate | A Rust binary or library.
dyn | Name the type of a trait object.
else | What to do when an if condition does not hold.
enum | A type that can be any one of several variants.
extern | Link to or import external code.
false | A value of type bool representing logical false.
fn | A function or function pointer.
for | Iteration with in, trait implementation with impl, or higher-ranked trait bounds (for<'a>).
if | Evaluate a block if a condition holds.
impl | Implement some functionality for a type.
in | Iterate over a series of values with for.
let | Bind a value to a variable.
loop | Loop indefinitely.
match | Control flow based on pattern matching.
mod | Organize code into modules.
move | Capture a closure's environment by value.
mut | A mutable binding, reference, or pointer.
pub | Make an item visible to others.
ref | Bind by reference during pattern matching.
return | Return a value from a function.
self | The receiver of a method, or the current module.
static | A place that is valid for the duration of a program.
struct | A type that is composed of other types.
super | The parent of the current module.
trait | A common interface for a class of types.
true | A value of type bool representing logical true.
type | Define an alias for an existing type.
union | The Rust equivalent of a C-style union.
unsafe | Code or interfaces whose memory safety cannot be verified by the type system.
use | Import or rename items from other crates or modules.
where | Add constraints that must be upheld to use an item.
while | Loop while a condition is upheld.

Author: bors