Fix the synopsis of pg_md5_hash

Hi,

The synopsis of pg_md5_hash() seems wrong such as:
    - s/int/bool/
    - "errstr" is missing
So, I created a patch to fix them.

src/common/md5_common.c
==================================================
*  SYNOPSIS      #include "md5.h"
*                int pg_md5_hash(const void *buff, size_t len, char *hexsum)
...
bool
pg_md5_hash(const void *buff, size_t len, char *hexsum, const char **errstr)
==================================================

Please find attached file.

Regards,
Tatsuro Yamada
NTT Open Source Software Center