clarify run-time requirements for numpy in migrator

[h-vetinari]

From discussion in conda-forge/numpy-feedstock#324

CC @hmaarrfk

[conda-forge-webservices]

Hi! This is the friendly automated conda-forge-linting service.

I just wanted to let you know that I linted all conda-recipes in your PR (recipe/meta.yaml) and found it was in an excellent condition.

[jakirkham]

Thanks Axel! 🙏

[hmaarrfk]

The whole paragraph:

Additionally, we no longer need to use {{ pin_compatible("numpy") }} as a
run requirement - this has been handled for more than two years now by a
run-export on the numpy package itself. The migrator will therefore remove
any occurrences of this.

should be adjusted

[h-vetinari]

I think the content of that paragraph is still correct, but I added a new one to clarify that removing pin_compatible does not mean that there shouldn't be run-requirements. Feel free to suggest ways to improve/shorten this.