info-apropos

`M-x info-apropos' does the same as `index-apropos' in the
standalone Info reader.  I'll just repeat from the Info manual:

 `M-x info-apropos'
      Grovel the indices of all the known Info files on your system
      for a string, and build a menu of the possible matches.

  If you don't know what manual documents something, try the `M-x
  info-apropos'.  It prompts for a string and then looks up that
  string in all the indices of all the Info documents installed on
  your system.