fix: highlight the required configuration and behavior of the fixer tools The problem is that `hg help fix` didn't mention *how* to configure the tools, and while I knew that `{rootpath}` existed in the configuration, I missed that the tools require reading content from stdin. (I configured `gofmt` to use `{rootpath}`, and that had the effect of squashing all changes in a file at `.` into the first commit and emptying that content from its descendants.) Basically all this does is put a pointer in the default (command level) help to the extension level help that mentions the configuration, and moves the extension level help that documents reading from stdin and writing to stdout to the top to give it more prominence. The last sentence is adjusted a bit to reflect the new location.