[doc] modifying unit from characters to bytes

Hi,

The manual describes the size of pg_stat_activity.query
as below:

  | By default the query text is truncated at 1024 characters;

When considering multibyte characters, it seems more
accurate to change the unit from "characters" to "bytes".

I also searched other "[0-9] characters" in the manual.
I may overlook something, but apparently it seems ok
because of their contexts which are limited to ASCII
character or other reasons.


Regards,

--
Atsushi Torikoshi
NTT DATA CORPORATION