Porter's Handbook: Document CARGO_USE_GITLAB

• Remove mention of CARGO_GH_CARGOTOML after ports r496795
• Update CARGO_USE_GITHUB description. It does not patch Cargo.lock. That is done automatically during the configure phase by cargo itself and is not worth mentioning here.

Approved by: mat
Differential Revision: https://reviews.freebsd.org/D19697