[Mac/iOS/GN] Document enable_stripping and enable_dsyms vs their GYP equivalents. BUG=330301 R=brettw@chromium.org Review-Url: https://codereview.chromium.org/2084153002 Cr-Original-Commit-Position: refs/heads/master@{#401320} Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src Cr-Mirrored-Commit: d6fd033cbaddd2f127b244c48a3cfd8f7a3e953d