Doc: do a little copy-editing on Index Storage Parameters list.

Doc: do a little copy-editing on Index Storage Parameters list.

Add a paragraph break per suggestion from David G. Johnston.
Use a consistent voice for all the different parameter
descriptions, and fix a couple of grammatical issues.

Reported-by: Igor Korot <ikorot01@gmail.com>
Co-authored-by: "David G. Johnston" <david.g.johnston@gmail.com>
Co-authored-by: Tom Lane <tgl@sss.pgh.pa.us>
Discussion: https://postgr.es/m/CA+FnnTz=EW1VQRpWB9J+G-NSchrPFcw4nR7d0JqzEK9jWKB35A@mail.gmail.com