external.md: clarify where external repositories are fetched to
external.md: clarify where external repositories are fetched to

Document that external repositories are handled on a per-workspace
basis and in the section on "fetching" link to the section explaining
the directory layout. Also, be more explicit about what symlinks
are created; symlinks are a way to make local repositories stay up
to date and are not to be confused with the convenience symlinks
that bazel creates.

Fixes #8091

Change-Id: I8b35d03abb5f4f8a1b429ee2cb875ec71060b9db
PiperOrigin-RevId: 273533845

aehlig/bazel,katre/bazel,cushon/bazel,bazelbuild/bazel,bazelbuild/bazel,meteorcloudy/bazel,katre/bazel,aehlig/bazel,aehlig/bazel,davidzchen/bazel,ulfjack/bazel,ulfjack/bazel,meteorcloudy/bazel,davidzchen/bazel,akira-baruah/bazel,dslomov/bazel-windows,perezd/bazel,ulfjack/bazel,dslomov/bazel,twitter-forks/bazel,werkt/bazel,werkt/bazel,safarmer/bazel,ButterflyNetwork/bazel,safarmer/bazel,perezd/bazel,akira-baruah/bazel,bazelbuild/bazel,akira-baruah/bazel,safarmer/bazel,safarmer/bazel,werkt/bazel,perezd/bazel,dslomov/bazel,twitter-forks/bazel,katre/bazel,twitter-forks/bazel,dslomov/bazel-windows,cushon/bazel,ulfjack/bazel,meteorcloudy/bazel,meteorcloudy/bazel,ButterflyNetwork/bazel,katre/bazel,cushon/bazel,dslomov/bazel-windows,meteorcloudy/bazel,perezd/bazel,dslomov/bazel-windows,bazelbuild/bazel,meteorcloudy/bazel,davidzchen/bazel,twitter-forks/bazel,ulfjack/bazel,cushon/bazel,twitter-forks/bazel,akira-baruah/bazel,ButterflyNetwork/bazel,akira-baruah/bazel,cushon/bazel,dslomov/bazel,dslomov/bazel-windows,dslomov/bazel,aehlig/bazel,bazelbuild/bazel,meteorcloudy/bazel,dslomov/bazel-windows,akira-baruah/bazel,aehlig/bazel,ButterflyNetwork/bazel,davidzchen/bazel,werkt/bazel,cushon/bazel,dslomov/bazel,perezd/bazel,davidzchen/bazel,aehlig/bazel,davidzchen/bazel,davidzchen/bazel,werkt/bazel,katre/bazel,ButterflyNetwork/bazel,twitter-forks/bazel,perezd/bazel,aehlig/bazel,bazelbuild/bazel,ulfjack/bazel,perezd/bazel,dslomov/bazel,safarmer/bazel,ButterflyNetwork/bazel,ulfjack/bazel,werkt/bazel,katre/bazel,safarmer/bazel,dslomov/bazel,twitter-forks/bazel