make help message string referencing more consistent and add a bit of readability

[kh0rvus]

• modified help message string storage and referencing for consistency
• grammar correction to docs/android/download_apks.md
• changed ios backup help message from a format string that would reference and explicitly print the environment variable, to printing the name of the environment variable itself

also you'll notice i violated line limits with some of the click.options, honestly just because it seemed more readable. if you're not a fan np i can wrap at 80.