Document the slight change in CRYPTO_mem_ctrl()

Reviewed-by: NKurt Roeckx <kurt@openssl.org>

doc/crypto/OPENSSL_malloc.pod

@@ -179,7 +179,8 @@ CRYPTO_set_mem_functions() and CRYPTO_set_mem_debug()
 return 1 on success or 0 on failure (almost
 always because allocations have already happened).
 
-CRYPTO_mem_ctrl() returns the previous value of the mode.
+CRYPTO_mem_ctrl() returns -1 if an error occured, otherwise the
+previous value of the mode.
 
 OPENSSL_mem_debug_push() and OPENSSL_mem_debug_pop()
 return 1 on success or 0 on failure.