Documentation for mps_arena_busy (and possibly elsewhere) does not define "normal" use of the MPS

[thejayps]

https://github.com/Ravenbrook/mps/pull/223/files#r1197864295

The above comment was noted during review of mps_addr_object, with @rptb1 commenting verbally during the review that the main reason for lack of clarity was the undefined use of the word "normal". We should examine other areas of documentation where this wording exists, such as for mps_arena_busy in arena.rst and add clarity if necessary.