Looks like some *roff/mandoc markup snuck into the tmux man-page: $ man tmux | grep -2 "Ar target" -l requests the clipboard from the client using the xterm(1) escape sequence. If Ar target-pane is given, the clipboard is sent (in encoded form), otherwise it is stored in a new paste buffer.
Based on my super-weak *roff/mandoc markup knowledge, that looks like it's supposed to be a macro to mark `target-pane` as an argument with .Ar and I don't know if it needs the parallel of an HTML/XML closing tag to mark the end. Figured I'd mention it since doc issues are normally treated as bugs. -tkc