Dear all,

When the user wants to add a new column (or edit an existing one), he
executes the command `org-columns-new'. Then he can enter or select a few
'column attributes' (see in manual https://orgmode.org/org.html#Column-attributes-1).
One of the attributes is "SUMMARY-TYPE". This attribute can be selected
from the list of very enigmatic symbols such as "+", "$", "X"
which do not convey any information to the user. So, I have prepared a
solution that will address this issue.

For each symbol, the 'docstring' of the function assigned to that symbol
will be displayed. For example: 
"+ -- Compute the sum of VALUES."  
"$ -- Compute the sum of VALUES, with two decimals."

What do you think about this solution? 
I also included a question in the commit message regarding the function
`org-columns--first-line-docstring' that I created.

Patch below: