reftex-cite-comment-format

reftex-cite-comment-format is a variable defined in `reftex-vars.el'.
Its value is

"%% %2a %y, %j %v, %P, %b, %e, %u, %s %<
"

Documentation:
Citation format used for commented citations. Must NOT contain %l.
See the variable `reftex-cite-format' for possible percent escapes.

You can customize this variable.