Docs: fix incorrect spelling of contrib/pgcrypto option.

pgp_sym_encrypt's option is spelled "sess-key", not "enable-session-key".
Spotted by Jeff Janes.

In passing, improve a comment in pgp-pgsql.c to make it clearer that
the debugging options are intentionally undocumented.

contrib/pgcrypto/pgp-pgsql.c

@@ -259,7 +259,10 @@ set_arg(PGP_Context *ctx, char *key, char *val,
 		res = pgp_set_convert_crlf(ctx, atoi(val));
 	else if (strcmp(key, "unicode-mode") == 0)
 		res = pgp_set_unicode_mode(ctx, atoi(val));
-	/* decrypt debug */
+	/*
+	 * The remaining options are for debugging/testing and are therefore not
+	 * documented in the user-facing docs.
+	 */
 	else if (ex != NULL && strcmp(key, "debug") == 0)
 		ex->debug = atoi(val);
 	else if (ex != NULL && strcmp(key, "expect-cipher-algo") == 0)

doc/src/sgml/pgcrypto.sgml

@@ -828,11 +828,11 @@ Applies to: pgp_sym_encrypt, pgp_pub_encrypt
   </sect4>
 
   <sect4>
-   <title>enable-session-key</title>
+   <title>sess-key</title>
 
    <para>
     Use separate session key.  Public-key encryption always uses a separate
-    session key; this is for symmetric-key encryption, which by default
+    session key; this option is for symmetric-key encryption, which by default
     uses the S2K key directly.
    </para>
 <literallayout>