WIP: Update partial_clone.md.

I am a bit new to this place, please forgive me for not following the documentation template.

The last version of this document mentioned "Support for --filter=sparse:oid using the clone command is incomplete". However, I followed the steps in this document and when arriving at step "git show master:shiny-app/.gitfilterspec", it gave me the error: "fatal: Invalid object name 'master'."

Considering the --filter option in the clone command did work for me, I figured I could update this document. Also I spent a good while on figuring out how to enable the uploadpack.allowFilter and uploadpack.allowAnySHA1InWant options, so I made an effort to make this more clear to other people.