Clear up documentation on how the extension searches for Black

[brettcannon]

@brettcannon That is definitely useful for Black, though of course this issue still remains for all other linters etc.

I notice the description for black-formatter.path in your link includes as examples both an executable path and a module name, so I suppose the word path is the plan going forward to refer to either an executable or a module name in all cases, contrary to @wpyoga's suggestions in microsoft/vscode-python#4922 (comment).

In your link, what does this mean:

The bundled black is only used if there is no installed version of black found in the selected python environment.

I have not tested this. Does the extension go hunting for black in various specific locations in your environment, or does it use some PATH setting in the user's environment, or do you have to specify the black-formatter.path setting? For the first two options, can that logic be used to hunt for linters etc generally (the main point of this GitHub issue)?

Originally posted by @jeremyn in microsoft/vscode-python#4922 (comment)

[luabud]

We made some updates to the the README clarifying that by default we use the Black formatter shipped with the extension, and how black-formatter.path can be customized to get the one from PATH. We're happy to make more clarifying changes if needed, just let us know by creating a new issue!