All I can find is this:
https://source.android.com/source/using-repo.html
But they seem to have focused on simplicity and accessibility, rather than deep, complete documentation of the calibre of man and vim.
For example,
I'd like to see every possible option to the repo sync command, and what it does, and including whether or not it accepts partial unambiguous flag names if you (intentionally or accidentally) don't type the whole --thin . :)
The source code to repo can be found here: https://android.googlesource.com/tools/repo.
I found it by googling google repo source
Updated to answer your question better:
Try
repo help
To get more information on a command and see what arguments it takes:
repo help sync
I also found documentation here: http://manpages.ubuntu.com/manpages/xenial/man1/repo.1.html
You can find documentation on many Linux commands by typing
<command> help
or
man <command>
Where <command>
is the actual command that you need more information on.