libssh2_userauth_publickey: add man page

I found an undocumented public function and we can't have it like
that. The description here is incomplete, but should serve as a template
to allow filling in...

docs/libssh2_userauth_publickey.3

@@ -0,0 +1,29 @@
+.\" $Id: template.3,v 1.4 2007/06/13 16:41:33 jehousley Exp $
+.\"
+.TH libssh2_userauth_publickey 3 "1 Jun 2007" "libssh2 0.15" "libssh2 manual"
+.SH NAME
+libssh2_userauth_publickey - authenticate using a callback function
+.SH SYNOPSIS
+#include <libssh2.h>
+
+.NF
+int libssh2_userauth_publickey(LIBSSH2_SESSION *session,
+                               const char *user,
+                               const unsigned char *pubkeydata,
+                               size_t pubkeydata_len,
+                               sign_callback,
+                               void **abstract);
+.SH DESCRIPTION
+Authenticate with the \fIsign_callback\fP callback that matches the prototype
+below
+.SH CALLBACK
+.nf
+int name(LIBSSH2_SESSION *session, unsigned char **sig, size_t *sig_len,
+         const unsigned char *data, size_t data_len, void **abstract);
+.fi
+
+This function gets called...
+.SH RETURN VALUE
+Return 0 on success or negative on failure.
+.SH SEE ALSO
+.BR libssh2_userauth_publickey_fromfile_ex(3)