Add documentation for elastic-agent-cert-key-passphrase option

Author: leehinman

docs/en/ingest-management/elastic-agent/configuration/env/shared-env.asciidoc

@@ -37,6 +37,15 @@ OPTIONAL INFO AND EXAMPLE
 
 // end::elastic-agent-cert-key[]
 
+// tag::elastic-agent-cert-key-passphrase[]
+|
+[id="env-{type}-elastic-agent-cert-key-passphrase"]
+`ELASTIC_AGENT_CERT_KEY_PASSPHRASE`
+
+| (string) The path to the file that contains the passphrase for the mutual TLS private key that {agent} will use to connect to {fleet-server}.  The file must only contain the characters of the passphrase, no newline or extra non-printing characters.
+
+// end::elastic-agent-cert-key-passphrase[]
+
 // =============================================================================
 
 // tag::elastic-agent-tag[]