Doc: improve description of pgbench script weights.
Point out the workaround to be used if you want to write a script
file name that includes "@". Clean up the text a little.
Fabien Coelho, additional wordsmithing by me
Discussion: https://postgr.es/m/
1c4e81550d214741827a03292222db8d@G08CNEXMBPEKD06.g08.fujitsu.local