docs: mention maximum precision of datetimes in concepts docs

[alkasm]

I could not find documentation which stated the allowed precision of datetimes. The docs link to the wiki article for iso8601, but the standard doesn't state a limit to the precision:

There is no limit on the number of decimal places for the decimal fraction. However, the number of decimal places needs to be agreed to by the communicating parties. For example, in Microsoft SQL Server, the precision of a decimal fraction is 3 for a DATETIME, i.e., "yyyy-mm-ddThh:mm:ss[.mmm]".

However if you look at the conjure master test cases, an example with 9 decimal digits is in the positive test set, and one with 10 digits in the negative set:

  - type: DateTimeExample
    positive:
        - '{"value":"2017-01-02T03:04:05Z"}'
        - '{"value":"2017-01-02T03:04:05.000Z"}'
        - '{"value":"2017-01-02T03:04:05.000000Z"}'
        - '{"value":"2017-01-02T03:04:05.000000000Z"}'  # 9 digits
        - '{"value":"2017-01-02T04:04:05.000000000+01:00"}'
        - '{"value":"2017-01-02T05:04:05.000000000+02:00"}'
    negative:
        - '{"value":"4/3/2018"}'
        - '{"value":"1523040070"}'
        - '{"value":1523040070}'
        - '{"value":"2017-01-02T03:04:05.0000000000Z"}'  # 10 digits
        - '{"value":"2017-01-02T04:04:05.000000000+01:00[Europe/Berlin]"}'

[palantirtech]

Thanks for your interest in palantir/conjure, @alkasm! Before we can accept your pull request, you need to sign our contributor license agreement - just visit https://cla.palantir.com/ and follow the instructions. Once you sign, I'll automatically update this pull request.