Syntax lookup: add ref value assignment documentation

[illusionalsagacity]

for #162

[bobzhang]

actually no sugar provided for ref, both ref and := are functions in pervasives module

[illusionalsagacity]

Gotcha, I was essentially quoting from pages/docs/manual/latest/mutation.mdx:

Assign a new value to `myValue` like so:

<CodeTab labels={["ReScript", "JS Output"]}>

\```res example
myValue.contents = 6
\```
\```js
myValue.contents = 6;
\```

</CodeTab>

We provide a syntax sugar for this:

<CodeTab labels={["ReScript", "JS Output"]}>

\```res example
myValue := 6
\```
\```js
myValue.contents = 6;
\```

</CodeTab>

Should I re-work this language as well?

[ryyppy]

I added some comments. I think it's not important how this is implemented, we should just describe what it does and how it is useful.

[ryyppy]

@illusionalsagacity looking good, thanks!