[DOCS] Clarify not all PKCS12 usable as truststores (#30750)

Although elasticsearch-certutil generates PKCS#12
files which are usable as both keystore and truststore
this is uncommon in practice. Settle these expectations
for the users following our security guides.

x-pack/docs/en/security/securing-communications/node-certificates.asciidoc

@@ -5,7 +5,7 @@
 TLS requires X.509 certificates to perform encryption and authentication of the
 application that is being communicated with. In order for the communication
 between nodes to be truly secure, the certificates must be validated. The
-recommended approach for validating certificate authenticity in a {es} cluster
+recommended approach for validating certificate authenticity in an {es} cluster
 is to trust the certificate authority (CA) that signed the certificate. By doing
 this, as nodes are added to your cluster they just need to use a certificate
 signed by the same CA and the node is automatically allowed to join the cluster.

x-pack/docs/en/security/securing-communications/tls-transport.asciidoc

@@ -30,9 +30,13 @@ See <<ssl-tls-settings, `xpack.ssl.verification_mode`>> for a description of the
 <2> If you created a separate certificate for each node, then you might need to
 customize this path on each node. If the filename matches the node name, you can
 use the `certs/${node.name}.p12` format, for example.
-<3> The `elasticsearch-certutil` output includes the CA certificate inside the
-PKCS#12 keystore, therefore the keystore can also be used as the truststore.
-This name should match the `keystore.path` value.
+<3> The `elasticsearch-certutil` outputs a PKCS#12 keystore which includes the
+CA certificate as a trusted certificate entry. This allows for the keystore to
+also be used as a truststore. In this case, the path value should match
+the `keystore.path` value.
+Note, however, that this is not the general rule. There are keystores that cannot be
+used as trustores, only 
+{ref}/security-settings.html#pkcs12-truststore-note[specifically crafted ones can]
 --
 
 ** If the certificate is in PEM format, add the following information to the

x-pack/docs/en/settings/security-settings.asciidoc

@@ -1231,6 +1231,19 @@ Password to the truststore.
 `xpack.ssl.truststore.secure_password` (<<secure-settings,Secure>>)::
 Password to the truststore.
 
+[[pkcs12-truststore-note]]
+[NOTE]
+Storing trusted certificates in a PKCS#12 file, although supported, is
+uncommon in practice. The {ref}/certutil.html[`elasticsearch-certutil`] tool,
+as well as Java's `keytool`, are designed to generate PKCS#12 files that
+can be used both as a keystore and as a truststore, but this may not be the
+case for container files that are created using other tools. Usually,
+PKCS#12 files only contain secret and private entries. To confirm that
+a PKCS#12 container includes trusted certificate ("anchor") entries look for
+`2.16.840.1.113894.746875.1.1: <Unsupported tag 6>` in the
+`openssl pkcs12 -info` output, or `trustedCertEntry` in the
+`keytool -list` output.
+
 [[http-tls-ssl-settings]]
 :ssl-prefix: xpack.security.http
 :component: HTTP