[Documentation] tap list returns more than just installed packages

[bryce-carson]

Steps to reproduce:

1. tap --help and note the below text.

   tap/tap/cfg.py

   Line 35 in d870474

   "list": "List installed packages",

2. tap list and observe that uninstalled (but known) packages are listed.

What I expected to happen:
tap list lists only packages that are installed.

---

tap list should only list installed packages, such as apt list --installed does.

[hiddeninthesand]

For what it's worth, tap list -im or longhand tap list --installed --mpr-only does this.

[bryce-carson]

Yep. Just clarifying the usage statement could be a small improvement.

Changing to "List packages" rather than "list installed packages" is another alternative, if the default is to list all packages.