Well, it is in the html help for WinBoard, and the man file for XBoard:
WinBoard html help wrote:/lgf or /loadGameFile filename
/lgi or /loadGameIndex N
If loadGameFile is set, WinBoard reads the specified game file at startup. You can leave out the name of this option and give just the file name, which is handy if you want to configure WinBoard as a game viewer with a browser such as the Windows Explorer or Netscape. The filename is interpreted relative to WinBoard's initial working directory. The filename "-" specifies the standard input. If there is more than one game in the file, WinBoard pops up a menu of the available games, with entries based on their PGN tags. If loadGameIndex is set to N, the menu is suppressed and the Nth game found in the file is loaded immediately. A game index of –1 will load the first game, but step to the next game on each new game in Match Mode. Similarly, an index of –2 will step to the next game every two games, so that all games are played by each player with both colors.
XBoard texi file wrote:If the loadGameIndex specifies an index -1, this triggers auto-increment
of the index in @code{matchMode}, which means that after every game the
index is incremented by one, causing each game of the match to be played
from the next game in the file. Similarly, specifying an index value of -2
causes the index to be incremented every two games, so that each game
in the file is used twice (with reversed colors).
The @code{rewindIndex} option causes the index to be reset to the
first game of the file when it has reached a specified value.
It is true that it is not mentioned in the WinBoard old WinHelp file, but we really should deprecate that, because almost no one can read that format anymore.