The config.cfg file is divided into several sections, each holds a number of different settings.
gfx
screen_w, screen_h Integer Width and height of virtual screen area (area of the screen bitmap the game writes on to)
window_w, window_h Integer Width and height of the actual window, or resolution for fullscreen modes
fullscreen Integer boolean Whether to use fullscreen graphics
color_depth Integer Bits per pixel of the virtual screen, may be 8, 16 or 32
use_vsync Integer boolean Use vsync to wait between frames, may make the game smoother on Windows and Linux
game
ticks_per_second Integer How many times your Loop function will be called, per second - FPS the game runs at
use_tick_timer Integer Attempts to use an internal timer to time frames precisely at the desired rate
window_title String Title of the window, if not fullscreen, default is "TAPEGRO game"
log_filename String If set, all errors and other developer console outputs will be written to this file
controls
player[player #]_device Integer 0 is keyboard, 1 is joystick, 2 is both Player # is 1 to 4
player[player #]_joystick Integer 0 is keyboard, number of the joystick used by this player, if there is only one joystick connected to the computer, it is always 0, and may be higher accordingly Player # is 1 to 4
player[player #]_keyboard_d[direction] Integer Scancode of key for directional controls Player # is 1 to 4, direction is a single char among u,d,l,r
player[player #]_keyboard_b[button #] Integer Scancode of key for button controls Player # is 1 to 4, button # is 0 to 11
player[player #]joystick_d Integer Joystick "stick" for directional controls, usually 0 will always be the DPAD Player # is 1 to 4
player[player #]joystick_b[button #] Integer Joystick button number Player # is 1 to 4, button # is 0 to 11
devconsole
enable Integer boolean Whether to enable the developer console (default is not to)
bgcolor Integer Color value of the background color for the developer console, default is 0, under 8 bit be careful to pick according to the palette
fgcolor Integer Color value of the foreground (text) color for the developer console, under 8 bit be careful to pick according to the palette
min_lines Integer Minimum number of lines the developer console buffer will contain
height Integer Height in (real not virtual) pixels
key Integer Scancode or string of key name of key to trigger opening the developer console
on_error Integer boolean Whether to automatically open the developer console on error
networking
driver Integer Network driver to use

You will also want to check the Allegro documentation's section "Configuration routines" which has at its bottom some Allegro-specific sections and configurations.
To be honest, the only useful ones are listed below:
sound
midi_card String If not set, the MIDI card will automatically detected - under MS-DOS, this is likely fine. However, under Windows this will pick the default awful sounding Microsoft synthesizer, and under Linux there may be no MIDI, so you may want to set this to "DIGI" - see the MIDI page.