Add information about `{more}`-Tag

[Zack-83]

The recently introduced {more} tag seems to be very useful, but the documentation reports nowhere how to use it.

How is the correct syntax to use it?

https://github.com/rdmorganiser/rdmo/blob/1cdcf9ea6c28c6e4398691ce3093726e79ae2dfb/rdmo/core/utils.py#L385

rdmorganiser/rdmo#1068

[MyPyDavid]

so far it is only documented as a code-comment:

# convert everything after `{more}` to <span class="more"><string></span> 
 to be shown/hidden on user input

https://github.com/rdmorganiser/rdmo/blob/79917de8dfdb8be0c988bc72f1352b307ce03cb1/rdmo/core/utils.py#L242