docs: update docs for new GitHub auth modes

Update documentation to include the new `browser` and `device`
authentication modes for GitHub. The old `oauth` value expands to both
`browser, device` for compatibility.

Author: mjcheetham

docs/configuration.md

@@ -185,7 +185,9 @@ If this option is not set, then the available authentication modes will be autom
 Value|Authentication Mode
 -|-
 _(unset)_|Automatically detect modes
-`oauth`|OAuth-based authentication
+`oauth`|Expands to: `browser, device`
+`browser`|OAuth authentication via a web browser _(requires a GUI)_
+`device`|OAuth authentication with a device code
 `basic`|Basic/PAT-based authentication
 
 #### Example

docs/environment.md

@@ -324,7 +324,9 @@ If this option is not set, then the available authentication modes will be autom
 Value|Authentication Mode
 -|-
 _(unset)_|Automatically detect modes
-`oauth`|OAuth-based authentication
+`oauth`|Expands to: `browser, device`
+`browser`|OAuth authentication via a web browser _(requires a GUI)_
+`device`|OAuth authentication with a device code
 `basic`|Basic/PAT-based authentication
 
 ##### Windows