🐛 doc: document sources of `syntax-theme` config option

[injust]

The documentation for --syntax-theme says that it defaults to the value of the BAT_THEME environment variable (if it contains a valid theme name).

I don't have BAT_THEME set. Instead, I configured bat by following catppuccin/bat's instructions, i.e. --theme="Catppuccin Mocha" in ~/.config/bat/config.

In order of descending priority, delta seems to use:

1. syntax-theme option in the git config file
2. BAT_THEME environment variable
3. theme option in the bat config file

The documentation should be updated to reflect this.

[l-armstrong]

Is this issue still open for me to work on it?

[injust]

@l-armstrong I haven't found time to work on this. If you'd like to contribute, please go for it!