group field tags
the required tag is actually optional
document remaining undocumented field tags
correct grammar errors and typos
rename SubCommand uniformly to Subcommand
remove periods at the end of field descriptions
Merge pull request #62 from zimmski/grammar-and-typos

Fix various spelling and grammar mistakes
start with a capital letter after a period
align field group descriptions
Merge pull request #61 from zimmski/undocumented-fields

Some field tags are undocumented