doc: Fix formatting of authentication.md
doc: Fix formatting of authentication.md

- Use consistent header formatting.
 - Replace inline triple backticks with single backticks.
 - Break overly long lines.

Closes #11445
deryni/cockpit,garrett/cockpit,garrett/cockpit,mvollmer/cockpit,cockpit-project/cockpit,cockpit-project/cockpit,moraleslazaro/cockpit,martinpitt/cockpit,cockpit-project/cockpit,martinpitt/cockpit,martinpitt/cockpit,moraleslazaro/cockpit,cockpit-project/cockpit,deryni/cockpit,moraleslazaro/cockpit,deryni/cockpit,moraleslazaro/cockpit,deryni/cockpit,mvollmer/cockpit,moraleslazaro/cockpit,mvollmer/cockpit,garrett/cockpit,cockpituous/cockpit,cockpituous/cockpit,cockpituous/cockpit,mvollmer/cockpit,garrett/cockpit,deryni/cockpit,moraleslazaro/cockpit,garrett/cockpit,cockpit-project/cockpit,martinpitt/cockpit,martinpitt/cockpit,deryni/cockpit,mvollmer/cockpit,moraleslazaro/cockpit,cockpituous/cockpit,cockpituous/cockpit,cockpituous/cockpit,deryni/cockpit