Bring FormattedTimestampStyle up to date with current docs

[inklesspen]

#2600 introduced the FormattedTimestamp struct and FormattedTimestampStyle enum.

However, since this PR was completed, apparently two new timestamp styles (s and S) were added.

Additionally, the doc uses different terms for some of the styles, compared to Serenity; what Serenity calls LongDateTime, Discord calls "Full Date, Short Time".

Style	Example Output	Description
t	16:20	Short Time
T	16:20:30	Medium Time
d	20/04/2021	Short Date
D	April 20, 2021	Long Date
f *	April 20, 2021 at 16:20	Long Date, Short Time
F	Tuesday, April 20, 2021 at 16:20	Full Date, Short Time
s	20/04/2021, 16:20	Short Date, Short Time
S	20/04/2021, 16:20:30	Short Date, Medium Time
R	2 months ago	Relative Time

[meditationmind]

Just for reference, this is the commit that added the new styles and changed the names. Seems like a pretty quick fix. Are you already working on a PR?

[inklesspen]

No, I'm busy with a few other things currently.

[nwerosama]

According to the commit that @meditationmind linked, they also changed the name on other styles which the PR #3496 would also update the names alongside adding s/S.