docs: fix markdown table example

[wagner-intevation]

the first table example (markup description) was wrong.

Also makes the explanation of the non-supported table alignment in rst clearer in what it means for this docs section

[RogerHaase]

Typo in:

**reST NOTE**: reST does not support cell alignment, 
therefore the last example shown does not the 
resulting alignment.

maybe:

**reST NOTE**: reST does not support cell alignment, 
therefore the last example shown above does not reflect the 
resulting alignment.

I would leave the start/end "|" characters in the first example because (currently) a markdown to markdown conversion yields start/end "|" characters for all tables.

Please resubmit with corrections.