Fix bug in VACUUM and ANALYZE docs

Hi,

Issue1:
VACUUM and ANALYZE docs explain that the parameter of BUFFER_USAGE_LIMIT 
is optional as follows. But this is not true. The argument, size, is 
required for BUFFER_USAGE_LIMIT. So the docs should be fixed this issue.
BUFFER_USAGE_LIMIT [ size ]
https://www.postgresql.org/docs/devel/sql-vacuum.html
https://www.postgresql.org/docs/devel/sql-analyze.html

Issue2:
Sizes may also be specified as a string containing the numerical size 
followed by any one of the following memory units: kB (kilobytes), MB 
(megabytes), GB (gigabytes), or TB (terabytes).
VACUUM and ANALYZE docs explain that the argument of BUFFER_USAGE_LIMIT 
accepts the units like kB (kilobytes), MB (megabytes), GB (gigabytes), 
or TB (terabytes). But it also actually accepts B(bytes) as an unit. So 
the docs should include "B(bytes)" as an unit that the argument of 
BUFFER_USAGE_LIMIT can accept.

You can see the patch in the attached file.

Ryoga Yoshida