Sidebar: can-cancel options only work if position is fixed

[xiaoxiao99]

As the title, for can-cancel option of sidebar, neither 'escape' nor 'outside' works if the position is not "fixed". I did not get that point until dig into the source code.
If this is by design, it may be better to add some notes in documentation.

Buefy version: [0.9.22]
Vuejs version: [2.7.13]
OS/Browser:
Monterey / Chrome