add documentation that SSH markers aren't supported

Cargo doesn't support the `@cert-authority` or `@revoked` markers in SSH
Known Hosts files. The lines are silently ignored.

If a user is depending on these lines to connect to a Git server via
SSH, then their command line Git client will work, but Cargo will fail
with an error that the host key doesn't match.

This change adds a note explaining that Cargo doesn't support these
markers and suggests that the user change their cargo configuration to
fetch with the CLI client instead.

Refs: rust-lang#11577

Author: hds

src/doc/src/appendix/git-authentication.md

@@ -81,6 +81,11 @@ publish their fingerprints on the web; for example GitHub posts theirs at
 Cargo comes with the host keys for [github.com](https://github.com) built-in.
 If those ever change, you can add the new keys to the config or known_hosts file.
 
+> **Note:** Cargo doesn't support the `@cert-authority` or `@revoked`
+> markers in `known_hosts` files. To make use of this functionality, use
+> [`net.git-fetch-with-cli`]. This is also a good tip if Cargo's SSH client
+> isn't behaving the way you expect it to.
+
 [`credential.helper`]: https://git-scm.com/book/en/v2/Git-Tools-Credential-Storage
 [`net.git-fetch-with-cli`]: ../reference/config.md#netgit-fetch-with-cli
 [`net.ssh.known-hosts`]: ../reference/config.md#netsshknown-hosts