fix(option): correct IsPresent IsAbsent docstrings

[mosiman]

The original docstrings for IsPresent and IsAbsent seemed wrong, or at the very least confusing. Let me know if I'm completely off base here!

[samber]

argh

thanks for the fix :-(

[codecov-commenter]

Codecov Report

Patch and project coverage have no change.

Comparison is base (d2b2b60) 88.98% compared to head (0b29bdd) 88.98%.

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Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #22 +/- ## ======================================= Coverage 88.98% 88.98% ======================================= Files 13 13 Lines 1080 1080 ======================================= Hits 961 961 Misses 107 107 Partials 12 12 ``` | Flag | Coverage Δ | | |---|---|---| | unittests | `88.98% <ø> (ø)` | | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Samuel+Berthe#carryforward-flags-in-the-pull-request-comment) to find out more. | [Impacted Files](https://codecov.io/gh/samber/mo/pull/22?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Samuel+Berthe) | Coverage Δ | | |---|---|---| | [option.go](https://codecov.io/gh/samber/mo/pull/22?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Samuel+Berthe#diff-b3B0aW9uLmdv) | `91.03% <ø> (ø)` | | Help us with your feedback. Take ten seconds to tell us [how you rate us](https://about.codecov.io/nps?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Samuel+Berthe). Have a feature suggestion? [Share it here.](https://app.codecov.io/gh/feedback/?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Samuel+Berthe)

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