magit-section-context-type: de-uglify
magit-prefix-p: improve doc-string

The old doc-string wasn't very precise, e.g. it left out that `equal'
is used for comparison.

Also rename arguments;  It just doesn't feel right to say.

  PREFIX is a prefix ... if ...

The hole point of this function is to check whether the first list is
a prefix of the second.  It might very well not be, so let's just call
it a list.
wip
very wip
wip