doc: specify the format of the `ca` tls option

doc/api/tls.markdown

@@ -104,9 +104,9 @@ automatically set as a listener for the [secureConnection][] event.  The
   - `cert`: A string or `Buffer` containing the certificate key of the server in
     PEM format. (Required)
 
-  - `ca`: An array of strings or `Buffer`s of trusted certificates. If this is
-    omitted several well known "root" CAs will be used, like VeriSign.
-    These are used to authorize connections.
+  - `ca`: An array of strings or `Buffer`s of trusted certificates in PEM
+    format. If this is omitted several well known "root" CAs will be used,
+    like VeriSign. These are used to authorize connections.
 
   - `crl` : Either a string or list of strings of PEM encoded CRLs (Certificate
     Revocation List)
@@ -262,9 +262,9 @@ Creates a new client connection to the given `port` and `host` (old API) or
   - `cert`: A string or `Buffer` containing the certificate key of the client in
     PEM format.
 
-  - `ca`: An array of strings or `Buffer`s of trusted certificates. If this is
-    omitted several well known "root" CAs will be used, like VeriSign.
-    These are used to authorize connections.
+  - `ca`: An array of strings or `Buffer`s of trusted certificates in PEM
+    format. If this is omitted several well known "root" CAs will be used,
+    like VeriSign. These are used to authorize connections.
 
   - `rejectUnauthorized`: If `true`, the server certificate is verified against
     the list of supplied CAs. An `'error'` event is emitted if verification