docs: add possible values to pgk-fmt argument

[azzamsa]

Fixes #378

TODO:

• [] How to prevent those values be out of sync with

  cargo-binstall/crates/binstalk/src/manifests/cargo_toml_binstall/package_formats.rs

  Line 9 in 757ecfa

  pub enum PkgFmt {

[azzamsa]

But, do you have any idea how to avoid out-out-sync with

cargo-binstall/crates/binstalk/src/manifests/cargo_toml_binstall/package_formats.rs

Line 9 in 757ecfa

pub enum PkgFmt {

?

The change in enum PkgFmt would require a manual change in the docstring.

[NobodyXu]

The change in enum PkgFmt would require a manual change in the docstring.

It's unfortunate, but with proper review, that should be OK since modifications to PkgFmt are rare.

[azzamsa]

The change in enum PkgFmt would require a manual change in the docstring.

It's unfortunate, but with proper review, that should be OK since modifications to PkgFmt are rare.

Oh. OK!