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How can I get "help" with vanilla sh builtins?

I discovered that the `-s` flag for `read` is a Bash-ism:

```
$ bash -c 'read -s -n 1' # waits until I press a key

$ sh -c 'read -s -n 1'
sh: 1: read: Illegal option -s
```

Vanilla `sh` does offer `read`:

```
$ sh -c 'type read'
read is a shell builtin
```

But I can't read `help` for the vanilla version:

```
$ help read # shows help for the Bash version
$ sh -c 'help read'
sh: 1: help: not found
$ man read # can only find the C system call, regardless of shell
```

This is *not* about the actual use of `read`. Rather: how can I read the manual properly?