You're right, it's not documented anywhere yet. Here's a complete list. Remember you have to check "Enable BBcode" in the object properties.
Text object
[b]bold text[/b]
[i]italic text[/i]
[u]underline text[/u]
[s]strikethrough text[/s]
[h2]change font size (in pt)[/size]
[font=Arial]change font face (including any webfont in project)[/font]
[color=#ff0000]change text color[/color]
[opacity=50]change text opacity[/opacity]
[hide]invisible text (useful for flashing effects)[/hide]
[background=#ff0000]change background color[/background]
[offsetx=10]offset text on X axis (useful for animated effects)[/offsetx]
[offsety=10]offset text on Y axis[/offsety]
[stroke]stroke text[/stroke]
[outline=#ff0000]outlined text[/outline]
SpriteFont object
[scalex=2]stretch wider[/scalex]
[scaley=2]stretch taller[/scaley]
[scale=2]stretch both axes[/scale]
[color=#ff0000]change text color[/color]
[opacity=50]change text opacity[/opacity]
[hide]invisible text (useful for flashing effects)[/hide]
[background=#ff0000]change background color[/background]
[offsetx=10]offset text on X axis (useful for animated effects)[/offsetx]
[offsety=10]offset text on Y axis[/offsety]
Tags can also be nested, e.g. text which is both bold and italic, text which is offset on both X and Y axes, etc.