Improve VACUUM/ANALYZE BUFFER_USAGE_LIMIT docs

Improve VACUUM/ANALYZE BUFFER_USAGE_LIMIT docs

This addresses various deficiencies in the documentation for VACUUM and
ANALYZE's BUFFER_USEAGE_LIMIT docs.

Here we declare "size" in the syntax synopsis for VACUUM and ANALYZE's
BUFFER_USAGE_LIMIT option and then define exactly what values can be
specified for it in the section for that below.

Also, fix the incorrect ordering of vacuumdb options both in the documents
and in vacuumdb's --help output.  These should be in alphabetical order.

In passing also add the minimum/maximum range for the BUFFER_USAGE_LIMIT
option.  These will also serve as example values that can be modified and
used.

Reported-by: Peter Eisentraut
Discussion: https://postgr.es/m/16845cb1-b228-e157-f293-5892bced9253@enterprisedb.com