Add serialisation output examples (#4286)

diff --git a/docs/hazmat/primitives/asymmetric/serialization.rst b/docs/hazmat/primitives/asymmetric/serialization.rst
index 24e2b89..90ec10e 100644
--- a/docs/hazmat/primitives/asymmetric/serialization.rst
+++ b/docs/hazmat/primitives/asymmetric/serialization.rst
@@ -418,12 +418,24 @@
         Frequently known as PKCS#1 format. Still a widely used format, but
         generally considered legacy.
 
+        A PEM encoded RSA key will look like::
+
+            -----BEGIN RSA PRIVATE KEY-----
+            ...
+            -----END RSA PRIVATE KEY-----
+
     .. attribute:: PKCS8
 
         A more modern format for serializing keys which allows for better
         encryption. Choose this unless you have explicit legacy compatibility
         requirements.
 
+        A PEM encoded key will look like::
+
+            -----BEGIN PRIVATE KEY-----
+            ...
+            -----END PRIVATE KEY-----
+
 .. class:: PublicFormat
 
     .. versionadded:: 0.8
@@ -443,11 +455,23 @@
         identifier and the public key as a bit string. Choose this unless
         you have specific needs.
 
+        A PEM encoded key will look like::
+
+            -----BEGIN PUBLIC KEY-----
+            ...
+            -----END PUBLIC KEY-----
+
     .. attribute:: PKCS1
 
         Just the public key elements (without the algorithm identifier). This
         format is RSA only, but is used by some older systems.
 
+        A PEM encoded key will look like::
+
+            -----BEGIN RSA PUBLIC KEY-----
+            ...
+            -----END RSA PUBLIC KEY-----
+
     .. attribute:: OpenSSH
 
         .. versionadded:: 1.4