Add some documentation for the OCSP callback functions

Describe the usage of the OCSP callback functions on both the client and
the server side.
Reviewed-by: NViktor Dukhovni <viktor@openssl.org>

doc/ssl/SSL_CTX_set_tlsext_status_cb.pod

+=pod
+
+=head1 NAME
+
+SSL_CTX_set_tlsext_status_cb, SSL_CTX_set_tlsext_status_arg,
+SSL_set_tlsext_status_type, SSL_get_tlsext_status_ocsp_resp,
+SSL_set_tlsext_status_ocsp_resp - OCSP Certificate Status Request functions
+
+=head1 SYNOPSIS
+
+ #include <openssl/tls1.h>
+
+ long SSL_CTX_set_tlsext_status_cb(SSL_CTX *ctx,
+                                   int (*callback)(SSL *, void *));
+ long SSL_CTX_set_tlsext_status_arg(SSL_CTX *ctx, void *arg);
+
+ long SSL_set_tlsext_status_type(SSL *s, int type);
+
+ long SSL_get_tlsext_status_ocsp_resp(ssl, unsigned char **resp);
+ long SSL_set_tlsext_status_ocsp_resp(ssl, unsigned char *resp, int len);
+
+=head1 DESCRIPTION
+
+A client application may request that a server send back an OCSP status response
+(also known as OCSP stapling). To do so the client should call the
+SSL_set_tlsext_status_type() function prior to the start of the handshake.
+Currently the only supported type is B<TLSEXT_STATUSTYPE_ocsp>. This value
+should be passed in the B<type> argument. The client should additionally provide
+a callback function to decide what to do with the returned OCSP response by
+calling SSL_CTX_set_tlsext_status_cb(). The callback function should determine
+whether the returned OCSP response is acceptable or not. The callback will be
+passed as an argument the value previously set via a call to
+SSL_CTX_set_tlsext_status_arg(). Note that the callback will not be called in
+the event of a handshake where session resumption occurs (because there are no
+Certificates exchanged in such a handshake).
+
+The response returned by the server can be obtained via a call to
+SSL_get_tlsext_status_ocsp_resp(). The value B<*resp> will be updated to point
+to the OCSP response data and the return value will be the length of that data.
+Typically a callback would obtain an OCSP_RESPONSE object from this data via a
+call to the d2i_OCSP_RESPONSE() function. If the server has not provided any
+response data then B<*resp> will be NULL and the return value from
+SSL_get_tlsext_status_ocsp_resp() will be -1.
+
+A server application must also call the SSL_CTX_set_tlsext_status_cb() function
+if it wants to be able to provide clients with OCSP Certificate Status
+responses. Typically the server callback would obtain the server certificate
+that is being sent back to the client via a call to SSL_get_certificate();
+obtain the OCSP response to be sent back; and then set that response data by
+calling SSL_set_tlsext_status_ocsp_resp(). A pointer to the response data should
+be provided in the B<resp> argument, and the length of that data should be in
+the B<len> argument.
+
+=head1 RETURN VALUES
+
+The callback when used on the client side should return a negative value on
+error; 0 if the response is not acceptable (in which case the handshake will
+fail) or a positive value if it is acceptable.
+
+The callback when used on the server side should return with either
+SSL_TLSEXT_ERR_OK (meaning that the OCSP response that has been set should be
+returned), SSL_TLSEXT_ERR_NOACK (meaning that an OCSP response should not be
+returned) or SSL_TLSEXT_ERR_ALERT_FATAL (meaning that a fatal error has
+occurred).
+
+SSL_CTX_set_tlsext_status_cb(), SSL_CTX_set_tlsext_status_arg(),
+SSL_set_tlsext_status_type() and SSL_set_tlsext_status_ocsp_resp() return 0 on
+error or 1 on success.
+
+SSL_get_tlsext_status_ocsp_resp() returns the length of the OCSP response data
+or -1 if there is no OCSP response data.
+
+=cut