doc: clean up deprecations.md doc.

[lholmquist]

This removes the --pending-deprecation label from the Buffer() constructor in the deprecation.md docs.

It was added originally, since it was just a Documentation only deprecation.
It is now a full runtime deprecation, which by default will output what
the flag does anyway, so it can probably be removed

While i was starting to add docs for #29671, i noticed this.

Checklist

• make -j4 test (UNIX), or vcbuild test (Windows) passes
• tests and/or benchmarks are included
• documentation is changed or added
• commit message follows commit guidelines

[BridgeAR]

This does not seem right to me. The buffer deprecation is special and it partially prints a runtime warning when the code is coming from outside of the node_modules.
Otherwise it only prints the warning with --pending-deprecation.

[jasnell]

I think then that the right thing to do is better clarify the use of --pending-deprecation here.

[Trott]

I think then that the right thing to do is better clarify the use of --pending-deprecation here.

I opened #29769 as an alternative but feel free to cherry-pick or copy in those changes and modify them to your liking.

[lholmquist]

@Trott going to close this in favor of yours, since it address the issue better. I didn't realize there was more to the flag for this particular example.