Clarify maximum score for problems

This PR was started because we noticed that the problem format allows both
-infinity and +infinity as maximum scores, but the Contest API doesn't allow
either. After talking to Fredrik and Nicky we decided not to 'fix' this:
- -infinity is being removed from the next version of the problem format.
- +infinity is rarely used, and there's an easy workaround (just pick an
  arbitrarily high value) so we'll leave it.

While discussing this we found that the max_score was poorly defined as
'this is the max, but maybe not' and decided to simplify and make it more
strict. I went with just removing the weak phrase: this IS the maximum
score possible.

I also felt 'indicate' could be improved (went with 'determine') and
'in scoring contests' was redundant (removed it).

As with anything in the spec, we'd expect servers to enforce the max_score,
but encourage clients to be liberal in what they accept. e.g. currently
the resolver doesn't work if you score the maximum and then have another
submission, but we'll fix this.

Author: deboer-tim

Contest_API.md

@@ -1133,7 +1133,7 @@ Properties of problem objects:
 | output\_limit     | integer   | Limit in MiB on what the submission can write both to `stdout` and `stderr`. If a submission produces more output, a CCS should fail the submission or ignore output beyond this limit.
 | code\_limit       | integer   | Limit in KiB on submissions for this problem. Submissions that are larger should be rejected by a CCS.
 | test\_data\_count | integer   | Number of test data sets.
-| max\_score        | number    | Maximum expected score, although teams may score higher in some cases. Typically used to indicate scoreboard cell color in scoring contests. Required iff contest:scoreboard\_type is `score`.
+| max\_score        | number    | Maximum score. Typically used to determine scoreboard cell color. Required iff contest:scoreboard\_type is `score`.
 | package           | array of FILE ? | [Problem package](https://www.kattis.com/problem-package-format/). Expected mime type is application/zip. Only exactly one package is allowed. Not expected to actually contain href for package during the contest, but used for configuration and archiving.
 | statement         | array of FILE ? | Problem statement. Expected mime type is application/pdf.