docs: minor fixes in the next/head doc (#70510)
### What?
Updated the documentation for `next/head` to clarify how `<title>` and
`<base>` tags are handled uniquely. Specifically, the documentation
states that these tags are automatically checked for duplicates by
Next.js and do not require the `key` attribute.
### Why?
The current documentation suggests using the `key` attribute to prevent
duplicate tags in the `head`. However, `<title>` and `<base>` tags are
automatically managed by the `unique` function in Next.js (`head.tsx`),
ensuring only one instance is rendered without needing a `key`. This
update provides clearer guidance and prevents potential
misunderstandings.
### How?
- Added a note explaining that `<title>` and `<base>` tags do not need a
`key` attribute as they are automatically checked for duplicates by
Next.js.
### Reference
Source code confirming this behavior:
[head.tsx#L85-L93](https://github.com/vercel/next.js/blob/3ead2d1fe98f78307861b12983c76d5a7a88c41a/packages/next/src/shared/lib/head.tsx#L84-L93)
<img width="741" alt="Window"
src="https://github.com/user-attachments/assets/a3a6f3fd-6aab-48a2-8d15-26df0692b0e1">
Co-authored-by: JJ Kasper <jj@jjsweb.site>
