doc/stdenv/stdenv: document runHook function (#341972)

}

```

### `runHook` \<hook\> {#fun-runHook}

Execute \<hook\> and the values in the array associated with it. The array's name is determined by removing `Hook` from the end of \<hook\> and appending `Hooks`.

For example, `runHook postHook` would run the hook `postHook` and all of the values contained in the `postHooks` array, if it exists.

### `substitute` \<infile\> \<outfile\> \<subs\> {#fun-substitute}

Performs string substitution on the contents of \<infile\>, writing the result to \<outfile\>. The substitutions in \<subs\> are of the following form: