docs: add flag package-lock-only for npm install

[m4rch3n1ng]

the flag is described in

cli/docs/lib/content/commands/npm-install.md

Lines 389 to 391 in 03aaa78

	The `--package-lock-only` argument will only update the
	`package-lock.json`, instead of checking `node_modules` and downloading
	dependencies.

and in params for npm ls and npm audit

cli/lib/commands/ls.js

Line 36 in 03aaa78

'package-lock-only',

cli/lib/commands/audit.js

Line 406 in 03aaa78

'package-lock-only',

but wasn't actually added to params for npm install