doc: correct it's vs. its usage

Fix a few instances where `it's` was being used where the possessive
`its` was called for. Some additional minor copy-editing of nearby text
(adding a comma and fixing a comma splice) was also performed.

PR-URL: https://github.com/nodejs/node/pull/10098


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>