Great!
Noted.
Yes you are right there.
The docs are, as forever, a work in progress 
Which commands work with which backends tends to be based on the feature flags. --inplace requires the PartialUploads feature flags on a backend and --track-renames requires Move or Copy.
If you want to see what features a backend has use rclone backend features remote:.
So some of this work could potentially be automated.
It might be worth a mention of --inplace on the local, ftp, sftp backends. I'd probably say something like "This backend can have partial uploads so normally rclone will upload to a .partial file. This can be disabled with --inplace.
As for --track-renames that should really be an option on the sync command itself, but isn't for historical reasons. Making it an option on the sync command would make it appear in the help properly and would mean it only worked on rclone sync.
Putting a link to the docs for --track-renames on the sync page would probably be a good idea. There are a whole pile of flags which are relevant to move/sync/copy/moveto/copyto which could have links too.
In the docs I try not to duplicate stuff as it makes maintaining the docs harder and I try to automate stuff which can be automated, so maybe this can be improved with a bit of automation. Maybe each command could have a list of flags that it would like the user to refer to and the doc creator could take that list, look up the help for each one and add them to the docs page.