docs: add clarity around SSL settings

[yaauie]

• Preface text adapted from Logstash Integration Filter (which operates in client-mode and therefore presents trust before identity, whereas this server-mode presents identity before trust)
• Adds details about the expected encodings and contents of certificate files

[edmocosta]

LGTM 🎉

[edmocosta]

Suggested change

	When communicating over SSL, this plugin can be configured to ask clients to present their own identities (see <<plugins-{type}s-{plugin}-ssl_client_authentication>>), and will fully-validate the presented proof-of-identity presented by the connecting client using the system trust store.
	When communicating over SSL, this plugin can be configured to ask clients to present their own identities (see <<plugins-{type}s-{plugin}-ssl_client_authentication>>), and will fully-validate the proof-of-identity presented by the connecting client using the system trust store.