clarify client certificate authentication limitation (#686)

Author: ms-henglu

docs/guides/service_principal_client_certificate.md

@@ -33,9 +33,11 @@ $ openssl x509 -signkey "service-principal.key" -in "service-principal.csr" -req
 Finally we can generate a PFX file which can be used to authenticate with Azure:
 
 ```shell
-$ openssl pkcs12 -export -out "service-principal.pfx" -inkey "service-principal.key" -in "service-principal.crt"
+$ openssl pkcs12 -certpbe PBE-SHA1-3DES -keypbe PBE-SHA1-3DES -export -macalg sha1 -out "service-principal.pfx" -inkey "service-principal.key" -in "service-principal.crt"
 ```
 
+~> **NOTE:** The certificate support in AzAPI provider has limitations, for example it can't decrypt keys in PEM format or PKCS#12 certificates that use SHA256 for message authentication. If you encounter such limitations, please generate the PFX file with above command.
+
 Now that we've generated a certificate, we can create the Azure Active Directory Application.
 
 ---

templates/guides/service_principal_client_certificate.md

@@ -33,9 +33,11 @@ $ openssl x509 -signkey "service-principal.key" -in "service-principal.csr" -req
 Finally we can generate a PFX file which can be used to authenticate with Azure:
 
 ```shell
-$ openssl pkcs12 -export -out "service-principal.pfx" -inkey "service-principal.key" -in "service-principal.crt"
+$ openssl pkcs12 -certpbe PBE-SHA1-3DES -keypbe PBE-SHA1-3DES -export -macalg sha1 -out "service-principal.pfx" -inkey "service-principal.key" -in "service-principal.crt"
 ```
 
+~> **NOTE:** The certificate support in AzAPI provider has limitations, for example it can't decrypt keys in PEM format or PKCS#12 certificates that use SHA256 for message authentication. If you encounter such limitations, please generate the PFX file with above command.
+
 Now that we've generated a certificate, we can create the Azure Active Directory Application.
 
 ---