topic: make command names valid as expected, even if ui.strict=true
Before this patch, "hg topics" and "hg stack" cause unintentional
"unknown command" failure, if ui.strict=true.
In such case, "topics [TOPIC]" and "stack [TOPIC]" are required as a
canonical name, because synopsis-like string is used as command name
pattern of @command annotation for them.
This patch also specifies additional "synopsis" for @command
annotation, for intentional output in online help of these commands.