Provide documentation for some state machine related functions

Reviewed-by: Richard Levitte <levitte@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/3051)

doc/man3/SSL_in_init.pod

@@ -0,0 +1,110 @@
+=pod
+
+=head1 NAME
+
+SSL_in_before,
+SSL_in_init,
+SSL_is_init_finished,
+SSL_in_connect_init,
+SSL_in_accept_init,
+SSL_get_state
+- retrieve information about the handshake state machine
+
+=head1 SYNOPSIS
+
+ #include <openssl/ssl.h>
+
+ int SSL_in_init(SSL *s);
+ int SSL_in_before(SSL *s);
+ int SSL_is_init_finished(SSL *s);
+
+ int SSL_in_connect_init(SSL *s);
+ int SSL_in_accept_init(SSL *s);
+
+ OSSL_HANDSHAKE_STATE SSL_get_state(const SSL *ssl);
+
+=head1 DESCRIPTION
+
+SSL_in_init() returns 1 if the SSL/TLS state machine is currently processing or
+awaiting handshake messages, or 0 otherwise.
+
+SSL_in_before() returns 1 if no SSL/TLS handshake has yet been initiated, or 0
+otherwise.
+
+SSL_is_init_finished() returns 1 if the SSL/TLS connection is in a state where
+fully protected application data can be transferred or 0 otherwise.
+
+Note that in some circumstances (such as when early data is being transferred)
+SSL_in_init(), SSL_in_before() and SSL_is_init_finished() can all return 0.
+
+SSL_in_connect_init() returns 1 if B<s> is acting as a client and SSL_in_init()
+would return 1, or 0 otherwise.
+
+SSL_in_accept_init() returns 1 if B<s> is acting as a server and SSL_in_init()
+would return 1, or 0 otherwise.
+
+SSL_in_connect_init() and SSL_in_accept_init() are implemented as macros.
+
+SSL_get_state() returns a value indicating the current state of the handshake
+state machine. OSSL_HANDSHAKE_STATE is an enumerated type where each value
+indicates a discrete state machine state. Note that future versions of OpenSSL
+may define more states so applications should expect to receive unrecognised
+state values. The naming format is made up of a number of elements as follows:
+
+B<protocol>_ST_B<role>_B<message>
+
+B<protocol> is one of TLS or DTLS. DTLS is used where a state is specific to the
+DTLS protocol. Otherwise TLS is used.
+
+B<role> is one of CR, CW, SR or SW to indicate "client reading",
+"client writing", "server reading" or "server writing" respectively.
+
+B<message> is the name of a handshake message that is being or has been sent, or
+is being or has been processed.
+
+Additionally there are some special states that do not conform to the above
+format. These are:
+
+=over 4
+
+=item TLS_ST_BEFORE
+
+No handshake messages have yet been been sent or received.
+
+=item TLS_ST_OK
+
+Handshake message sending/processing has completed.
+
+=item TLS_ST_EARLY_DATA
+
+Early data is being processed
+
+=item TLS_ST_PENDING_EARLY_DATA_END
+
+Awaiting the end of early data processing
+
+=back
+
+=head1 RETURN VALUES
+
+SSL_in_init(), SSL_in_before(), SSL_is_init_finished(), SSL_in_connect_init()
+and SSL_in_accept_init() return values as indicated above.
+
+SSL_get_state() returns the current handshake state.
+
+
+=head1 SEE ALSO
+
+L<ssl(7)>,
+L<SSL_read_early_data(3)>
+
+=head1 COPYRIGHT
+
+Copyright 2017 The OpenSSL Project Authors. All Rights Reserved.
+
+Licensed under the OpenSSL license (the "License").  You may not use
+this file except in compliance with the License.  You can obtain a copy
+in the file LICENSE in the source distribution or at
+L<https://www.openssl.org/source/license.html>.
+
+=cut